I just wanted to add this solution to this great support forum
I was having a problem with Sub Menus showing under the Page Menu when I added a new page as a child of one of the main pages. I already had a number of pages as the child, that were not showing. But since I upgraded to 3.6.4 (from 3.4.9) when I added a page it was showing as a sub-menu. I was tearing my hair out and about to post here but found the solution...
To explain better the problem was as follows. I have a photography website with a number of pages in the following format
All of the existing children of GALLERY did not show as a sub-menu. But when I added a new page today, with gallery as the parent this one page would show as a 'pull down' sub menu under gallery.
Finally I found the solution. Under ATO->Header Area Menu 1 there is an option called "Depth of Page Menu Bar". This was defaulted to 0. Changing this to '1' changed it so that only HOME / GALLERY etc were showing with no children.
No doubt a good extra bit of functionality but it caught me out for a few hours as the old pages were working as expected! Hope this helps someone.