具有特定主题位置的wp_nav_menu不可见

In my Wordpress application, wp_nav_menu is not visible. The other menu works as intended.

This is invisible:

<?php wp_nav_menu( array('theme_location' => 'mobile_menu', 'container_class'=>'my-mobile-wrapper')); ?>

This is visible (both are in header.php)

<?php wp_nav_menu( array('theme_location' => 'header_menu', 'container_class'=>'header-menu')); ?>

I read a suggestion from somewhere, but adding this to functions.php didn't work.

add_action( 'pre_get_posts', 'my_pre_get_posts' );
    function my_pre_get_posts($query) {
        if ($query->get('post_type') === 'nav_menu_item') {
            $query->set('tax_query','');
        }
    }

I register as follows:

if (!function_exists('my_register_menus')) {
    function my_register_menus() {

        register_nav_menus(
            array(
                'header_menu' => __('Header Menu', "my"),
                'mobile_menu' =>  __('Mobile Menu', "my")
            )
        );

    }
}

add_action( 'after_setup_theme', 'my_register_menus' );

Thanks by now.

OK I found the answer.

I use a function to extend menu contents and I do a structural mistake at that point. I write here for the ones who might face a similar problem:

add_filter( 'wp_nav_menu_items', 'my_extend_menu_content', 10, 2 );

function my_extend_menu_content($items, $args){
    switch .... for menus other than mobile.

    // After the switch block:
    return false; // which is wrong.
    return $items; // should be this way.
}