On your mobile menu, by default, you'll notice that in order for the drop-down menu to reveal, you'll need to tap on the arrow symbol:
If you would like the drop-down menu to also be revealed by tapping on the "parent" menu link (the top level link), you'll need to turn it into a dead-link.
You can head into Online Store > Navigation. In your Main Menu choose the link you would like to make into a dead link and click edit. In the "Link" field, add an "#" So it looks something like this:
Then, tapping on the "parent" menu item will also reveal the drop-down menu on mobile: