add_submenu_page not working

I have a built a plugin which creates a custom menu like this:

    add_menu_page( 'Wholesale Pricing', 'Wholesale', 'manage_options', 'woo-wholesale', 'woo_wholesale_page_call'); 

I am now trying to add a subpage item underneath this. I have read the codex and came up with:

add_submenu_page( 'woo-wholesale', 'Registrations', 'Registrations', 'manage_options', 'woo-wholesale-registrations', 'wwpr_page_call' ); 

I am guessing this is incorrect as the submenu item isnt showing. Can anyone shed any light on this please?

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.