Mobile menu sub pages not accessible

For nav menu, both parent and sub pages work fine on a larger screen, but when shifting to a mobile size viewport, only the parent level menu items are accessible: The parent item shows a small drop-down arrow suggesting sub pages, but doesn’t actually drop down to reveal anything. The only thing that is clickable is the parent item.

Any idea why this is happening.

I haven’t altered the css in any way.

The site is an internal server build so I can’t show it to you.