What theme are you using? Where did you download it from?
I didn’t download it. It was a custom made theme from a web-developer for a company.
I’m sorry but we cannot really help with custom themes as we have no idea how they were constructed. I can only suggest that you contact the original developers of the theme or try hiring someone to look into this for you.
Is there any general vaque suggestion you can give me? Or an example of how it would be done in a non-custom theme?
In a WPORG theme, support for custom menus is mandatory. And even when no custom menu has been set up, those themes that do have Page menus will show all top-level Pages in the default menu automatically. Some even have dropdowns to show sub-Pages automatically. Perhaps yours is designed to show only top-level pages? You could try reviewing http://codex.wordpress.org/Function_Reference/wp_list_pages and http://codex.wordpress.org/Function_Reference/wp_page_menu