The megamenu is not working for the menus on the right side that have grandchildren menus because the menus are not displaying within the browser window. They are not aware that the browser window has ended somehow.
For an example try navigate go to the menu:
Shop -> Amazon Storefront -> Books -> Nutrition & Diet
on a window that is 1024 pixels wide you won't be able to get to the Nutrition & Diet menu item. It will be hidden from view. The only way I can see it is by using a two monitor system and opening my browser window wide enough to be visible across both monitors.