add_submenu_page not working

Make sure that your add_action hook is set to admin_menu.

Here’s a sample code:

add_action('admin_menu', 'wpse149688');
function wpse149688(){
    add_menu_page( 'Wholesale Pricing', 'Wholesale', 'manage_options', 'woo-wholesale', 'woo_wholesale_page_call');
    add_submenu_page( 'woo-wholesale', 'Registrations', 'Registrations', 'manage_options', 'woo-wholesale-registrations', 'wwpr_page_call' ); 
}

Also check whether user you’ve logged in as has the ability to view this menu. As this menu is set using manage_options capability.