As a primary link, a Custom Page won't expand

First, this is a wonderful module. It will take Drupal theming to a whole new level, IMHO.
Now my question: I created a page called MyPage, and created mypage.tpl.php. MyPage is one of my primary links, and shows up in my Primary links menu. I added some other pages below it, that is, as its children. So far so good. But here is the problem: When MyPage is clicked in the menu, it doesn't expand and show its children.
Oddly, if I set MyPage to "expanded" in admin/build/menu-customize/primary-links, the menu does correctly show MyPage's children. I suspect this may be an issue with Drupal menus and /or primary links, but I'd welcome any thoughts.