All pages MUST have a parent – otherwise they appear in the main menu and i would not like that to grow without good cause.
All pages have, tucked away in the Page Attributes section of the Document inspector, an Order. This defaults to zero. Use this number to set the order in which items appear in the menu. Without that, the order may be arbitrary. The order is not site-wide, it is only used in each submenu listing. It may be a good idea to use ‘BASIC’ numbering and increment in tens to make insertions easier. The Order field is also available in the page listing screen of the Admin dashboard if you hover a page title and select ‘Quick Edit’. Save a lot of time. The default page listing order in the admin screen shows the hierarchy of pages.
If you add or edit pages and expect the menu to update, look for the ‘Sync Auto Menu’ at the very top of the screen. Only when this has been changed will the menus update.