Editing menus in twenty ten theme

Hi I am using 3.8 and twenty ten theme and am having problems with Submenus.

Initially I thought it was just a problem adding new items to an existing menu (by dragging them to the right and saving), but I don’t seem to be able to add any new submenu items at all, and if I remove existing submenu items I can’t put them back!

I have also tried creating a brand new menu, and this won’t let me create ANY submenus…

It matters not whether the pages are correctly assigned as parent or child pages, and I’ve tried disabling all plugins and changing the theme to each of the default themes and I still cannot create any new sub items with any theme…

This is very strange and quite frustrating, I would really appreciate some help please!!