Hi,
When the screen is smaller than the default resolution, the responsive theme creates a "second floor" menu.
When a box-menu of some menu is open, and you hover on the box but behind has an item of main menu, the box is closed.
http://i.imgur.com/gjhtRyI.png
In the image that's linked above. When you hover the pointer on the "football" sub-categorie, the box is closed, cuz "contact us" item is behind.
In my store we have lots of categories in the main menu, so the "second floor" 's boxes are always closed by this bug. Making the boxes inaccessible.