You will need to create a custom menu do drop-downs to the sub-pages from parent pages appear in your navigation. These are the how-to instructions:
1. You create a custom menu. http://en.support.wordpress.com/menus/
2. You add the Categories you want to display and arrange them in the order you want.
3. Then you add and drop and drag the sub-categories below the appropriate Categories, and when you have everything arranged in the menu that you want you save the menu.
4. Go to the “theme location” module at upper left on the menu page. Select your custom menu name from the pull-down labeled “primary location.” Click the save button in that module.
5. Refresh your browser so that it isn’t possibly pulling a cached page, and view see your new menu with Parent pages and dropdowns to sub-pages.
Note: It is important that there is consistency in the sub-page settings. If you have difficulty with positioning using the drop and drag method then do check the sub-pages themselves to see if you have established a Parent page and child page “hierarchical structure” in the page attributes module. Remove if required. Also remove any “order” numbers you have assigned that may conflict with the order you are creating by dropping and dragging. Or simply change them so they are the same order as you establish in the menu. Establish consistency – problem solved.