I'm having a terrible time figuring this out. it seems that some themes do allow drop-down menus here, while others don't. I'm also starting to suspect that the menu chosen for 'navigation primary menu', the one that ends up as the main menu beneath the header image, doesn't always allow drop-downs even if it did before.
For instance, on my old site, I had a drop-down menus (using theme 'Parament'). Now that I'm running a different site with Parament, even though my site has 3 categories with 2 sub-categories, parament just displays all 5 categories side-by-side in my main menu, w/o consideration that some are drop-down!
What am I missing?? Are drop-down menus only capable of sub-categories for pages, but not posts??