I'm using pukeko theme to build a demo site; I created a sub menu in the main navigation, that works good for large screens.
But if you test the site in "phone mode" the sub menu disappears: the caret is clickable but nothing happens.
A plugin might be the culprit here, see this (older) thread.
Is the site publicly accessible? It might be interesting to take a closer look at the behavior that you have described.
thank you Thomas, but it was my fault: I had an extra css with overwriting rules that messed things up.
Ah, good to know!