I'm pretty sure there is a bug in the theme that is preventing this as I am experiencing the same issue. I have noticed if I remove my custom menus from the menu choser, some of the menus them appear and the dropdowns funtion. If I add it back, they cease working. If I switch to another theme, dropdown menus work as expected.
I submitted a support question to the developer 2 days ago and am awaiting a reply. With the weekend and holiday, I'm not surprised to have not received an answer yet. I hope I can get this working as it is a really nice theme!
If I get a resolution, I'll post it here.