Hello,
If your sub-menu items are mega menu blocks, mobile does not support mega menu blocks as there is no space to show them.
You need to create a separate menu without mega blocks, however, you can make a multy-level menu: menu and sub-menu items. You can find the instruction here: https://codex.wordpress.org/WordPress_Menu_User_Guide
If your mobile menu does not support a multi-level menu, please provide site admin access to the private area, we will check the settings.
Make the full backup of your site and confirm the permit for us to deactivate the plugins not related to the theme and switch to the parent theme for the while of checking what may take up to 15 minutes.
Best Regards
.
