I was looking for the same and found the work around as
1. Go to Appearance -> Menus
2. Go to top right corner for Screen Options & click open the pan.
3. Check on CSS Classes
4. Open the menu you want Dropdown (parent menu)
5. You will find a CSS Classes (optional)
6. Enter following text there has-flyout
7. Save the menu.
I was facing the exact same issue. I am not using any child-theme currently; just Reverie directly.
I tried the fix vikcon mentioned above but it only gives me a drop-down arrow with that menu item, while nothing actually drops down. I can confirm that I do have an item (a sub-category of that category) added under that menu item (the main category). It just doesn’t show up on hover in any sort of drop-down.
Any help regarding getting the dropdown will be greatly appreciated.
PS: I haven’t edited the menu part of the theme; I have only added YOAST Breadcrumb navigation code below it.