Mega Menu – items disappearing as I add new product catagories to the menu

I am trying to create mega menus as a way of navigating my site. Is there a limit on how much you can add?

When I add product categories to the menu and save, the following happens,

1. The menu location has to be reset in “manage locations” because the box becomes unchecked

2. Whatever I have added results in another category been deleted from the mega menu.