El cuadro de búsqueda de wp_list_table no se muestra

Estoy usando WordPress 3.8 y una clase personalizada derivada de WP_List_Table. ¿Por qué no aparece el formulario de búsqueda?

Mi código en la clase del plugin:

function __construct()
{
    register_activation_hook( __FILE__, array( $this, 'swb_plugin_install' ) );
    register_deactivation_hook( __FILE__, array( $this, 'swb_plugin_deactivate' ) );
    register_uninstall_hook( __FILE__, array( $this, 'swb_plugin_remove' ) );

    add_action( 'wp_head', array( $this, 'swb_header' ) );
    add_action( 'admin_menu', array( $this, 'swb_admin_page' ) );
    add_action( 'admin_enqueue_scripts', array( $this, 'swb_admin_script' ) );
}

function swb_admin_resellers()
{
    $list_obj = new SWB_Resellers_List_Table();
    if( isset($_POST['s']) ){
        $list_obj->prepare_items($_POST['s']);
    } else {
        $list_obj->prepare_items();
    }
    echo '<div class="wrap"><h2>Lista de Revendedores</h2>';
    ?>
    <form method="post">
        <input type="hidden" name="page" value="<?php echo $_REQUEST['page'] ?>" />
        <?php $list_obj->search_box('buscar revendedor(es)', 'search_id'); ?>
        <?php $list_obj->display(); ?>
    </form></div>
    <?php
}

function swb_admin_page()
{
    add_menu_page( '', 'Smarts Web Builder', 'activate_plugins', 'swb_plugin_root', array( $this, 'swb_admin_settings' ) );
    add_submenu_page( 'swb_plugin_root', 'Configuración General SWB', 'Configuración General', 'activate_plugins', 'swb_plugin_root', array( $this, 'swb_admin_settings' ) );
    add_submenu_page( 'swb_plugin_root', 'Lista de Revendedores SWB', 'Revendedores', 'activate_plugins', 'swb_resellers', array( $this, 'swb_admin_resellers' ) );
    add_submenu_page( 'swb_plugin_root', 'Lista de Licencias SWB', 'Licencias', 'activate_plugins', 'swb_licenses', array( $this, 'swb_admin_licenses' ) );
    add_submenu_page( 'swb_plugin_root', 'Lista de Clientes SWB', 'Clientes', 'activate_plugins', 'swb_customers', array( $this, 'swb_admin_customers' ) );
    add_submenu_page( 'swb_plugin_root', 'Temas SWB', 'Administrar Temas', 'activate_plugins', 'swb_themes', array( $this, 'swb_admin_themes' ) );
    add_submenu_page( 'swb_plugin_root', 'Plugins SWB', 'Administrar Plugins', 'activate_plugins', 'swb_plugins', array( $this, 'swb_admin_plugins' ) );
}

La clase que extiende WP_List_Table:

if(!class_exists( 'SWB_Resellers_List_Table' ) )
{
    class SWB_Resellers_List_Table extends WP_List_Table
    {
        function __construct()
        {
            parent::__construct(array(
                'singular' => 'wp_list_text_link',
                'plural' => 'wp_list_test_links',
                'ajax' => false
            ));
        }

        function get_columns()
        {
            return $column = array(
                'col_name' => __('Nombre Real'),
                'col_username' => __('Nombre de Usuario'),
                'col_contact_no' => __('Número de Contacto'),
                'col_email' => __('Email'),
                'col_active' => __('¿Está Activo?'),
                'col_registered_time' => __('Registrado'),
                'col_modified_time' => __('Última Modificación')
            );
        }

        public function get_sortable_columns()
        {
            return $sortable = array(
                'col_name' => array(
                    'name',
                    false
                ),
                'col_username' => array(
                    'username',
                    false
                ),
                'col_email' => array(
                    'email',
                    false
                ),
                'col_active' => array(
                    'active',
                    false
                ),
                'col_registered_time' => array(
                    'registered_time',
                    false
                ),
                'col_modified_time' => array(
                    'modified_time',
                    false
                )
            );
        }

        function prepare_items($search='')
        {
            global $wpdb;

            $table = $wpdb->prefix . SWB_TABLE_RESELLER;
            $join1 = $wpdb->prefix . "wlm_user_options";
            $join2 = $wpdb->users;

            $short_table = "T1";
            $short_join1 = "T2";
            $short_join2 = "T3";

            $column = "{$short_table}.reseller_id reseller_id,
                {$short_table}.user_id user_id,
                {$short_table}.active active,
                {$short_table}.modified_time modified_time,
                (SELECT {$short_join1}.option_value FROM {$join1} {$short_join1} WHERE {$short_join1}.option_name='custom_real_name' AND {$short_join1}.user_id={$short_table}.user_id) name,
                (SELECT {$short_join1}.option_value FROM {$join1} {$short_join1} WHERE {$short_join1}.option_name='custom_contact_no' AND {$short_join1}.user_id={$short_table}.user_id) contact_no,
                {$short_join2}.user_login username,
                {$short_join2}.user_email email,
                {$short_join2}.user_registered registered_time";
            $on1 = "{$short_join2}.ID={$short_table}.user_id";

            $where = "";
            $where .= !empty($_POST['name']) ? "name=" . trim($_POST['name']) : null;
            $where .= !empty($_POST['username']) ? "username=" . trim($_POST['username']) : null;
            if(!empty($search)){
                if( !empty($where) )    $where .= " AND ";
                $where = "(name LIKE '%{$search}%' OR username LIKE '%{$search}%')";
            }
            if( !empty($where) )    $where = " WHERE " . $where;

            $ordercolumn = !empty($_GET['orderby']) ? trim($_GET['orderby']) : null;
            $orderdirection = !empty($_GET['order']) ? trim($_GET['order']) : null;
            $orderby = "{$ordercolumn} {$orderdirection}";
            if( !empty($orderby) )  $orderby = "ORDER BY " . $orderby;

            $query = "SELECT {$column} FROM {$table} {$short_table} JOIN {$join1} {$short_join1} JOIN {$join2} {$short_join2} ON {$on1}{$where}{$order_by}";

            $totalitems = $wpdb->query($query);

            $perpage = 10;

            $paged = !empty($_GET["paged"]) ? intval($_GET["paged"]) : '';

            if (empty($paged) || !is_numeric($paged) || $paged <= 0) {
            $paged = 1;
            }

            $totalpages = ceil($totalitems / $perpage);

            if (!empty($paged) && !empty($perpage)) {
            $offset = ($paged - 1) * $perpage;
            $query .= ' LIMIT ' . ( int ) $offset . ',' . ( int ) $perpage;
            }

            $this->set_pagination_args(
                array(
                    "total_items" => $totalitems,
                    "total_pages" => $totalpages,
                    "per_page" => $perpage
                )
            );

            $columns               = $this->get_columns();
            $hidden                = array();
            $sortable              = $this->get_sortable_columns();
            $this->_column_headers = array(
                $columns,
                $hidden,
                $sortable
            );

            $this->items = $wpdb->get_results($query);
        }

        function display_rows()
        {
            $records = $this->items;
            $columns = null;
            $hidden  = null;
            list($columns, $hidden) = $this->get_column_info();

            if (!empty($records)) {
                foreach ($records as $rec) {
                    echo '<tr id="swb-reseller-row-' . $rec->reseller_id . '">';
                    foreach ($columns as $column_name => $column_display_name) {
                    $class = "class='{$column_name} column-{$column_name}'";
                        $style = "";
                        if (in_array($column_name, $hidden))
                            $style = ' style="display:none;"';
                        $attributes = $class . $style;

                        switch ($column_name) {
                            case "col_name":
                                echo '<td ' . $attributes . '>' . stripslashes($rec->name) . '</td>';
                                break;
                            case "col_username":
                                echo '<td ' . $attributes . '>' . stripslashes($rec->username) . '</td>';
                                break;
                            case "col_contact_no":
                                echo '<td ' . $attributes . '>' . stripslashes($rec->contact_no) . '</td>';
                                break;
                            case "col_email":
                                echo '<td ' . $attributes . '>' . stripslashes($rec->email) . '</td>';
                                break;
                            case "col_active":
                                $active = ( intval($rec->active) == 1 ) ? 'Sí' : 'No';
                                echo '<td ' . $attributes . '>' . $active . '</td>';
                                break;
                            case "col_registered_time":
                                echo '<td ' . $attributes . '>' . date("F j, Y g:i a", stripslashes($rec->registered_time)) . '</td>';
                                break;
                            case "col_modified_time":
                                echo '<td ' . $attributes . '>' . date("F j, Y g:i a", stripslashes($rec->modified_time)) . '</td>';
                                break;
                        }
                    }
                    echo '</tr>';
                }
            }
        }
    }
}

No sé por qué no aparece el cuadro de búsqueda. ¿Qué me falta?