Sub menu not displaying in Bartik Theme
The main menu in Bartik only displays the first level.  Since it's hardcoded in page.tpl.php there's nothing you can do about it.

Here's how to get the menu to work properly:


In your theme settings (admin/appearance/settings/barti) disable "Main menu"
Go to admin/structure/block and add the "Main Menu" block to the header region (or wherever you want it)


Alternatively, you can also check out the excellent Menu Block module, which generates extra customizable blocks (1st level, 2nd level, etc) for each menu.