Doesnt show set label

Hello, the plugin works ok in my localhost, however in my live site it doesnt show the set label, it only shows the box for the new featured image but it doesnt let me assign the featured image.
Any help would be welcome.
Best regards

/*multiple featured images */
if( class_exists( 'kdMultipleFeaturedImages' ) ) {

        $args = array(
                'id' => 'imagen-post-secundario',
                'post_type' => 'post',      // Set this to post or page
                'labels' => array(
                    'name'      => 'imagen-post-secundario',
                    'set'       => 'Establecer imagen',
                    'remove'    => 'Eliminar imagen',
                    'use'       => 'Usar como imagen',
                )
        );
        new kdMultipleFeaturedImages( $args );
}

http://wordpress.org/plugins/multiple-featured-images/