Adding submenus to PopUpMenu

I’m trying to get my head around the PopUpMenu, one thing that confuses me is how to effectively add subMenus to subMenus. If I add a subMenu to a parent and then another subMenu to the first subMenu only the first subMenu is displayed. If I attach subMenu to subMenu and then add the first to the parent it displays as expected.

I was hoping to add subMenus from the menu itself, but this behavior complicated the matter. Should I rebuild the entire menu for each subMenu added?

Any insight would be appreciated