I am using the Twenty Eleven theme (version 1.2. I haven't updated as it tells me that all customisation will be lost if I do).
I organised my menu so that some of the pages are a sub-menu of another. The sub-menu shows up fine as a dropdown when hovering over the parent page, but the moment you move the mouse down, to click on one of the sub-menu options, it disappears. So in effect, the sub-menu isn't working, even though WordPress seems to have recognised that it supposed to be a sub-menu.