Drop down menu doesn’t work

I’m pretty familiar with WP and, until now, haven’t had problems creating a top-bar drop down menu. I’m trying to create a dropdown of 5 submenus under the “what I do” (services) menu item in the main nav menu. I set it up, as usual, while I created the menu — placing the submenu items below the “parent” item. I also designated each submenu item as having that “parent” during page creation. Nothing works. Please advise.