This is because you are adding button on one page and when you open a another page these menu buttons are not available. If these are navigation items than I will suggest to add it to project navigation as shown in the screenshot below. If you want to have these as button or custom navigation them make sure you add this to the main layout or on every page. (I will suggest to add it on main layout).
I am not sure if you have already had a look at learning paths in Mendix academy, if not than I will highly recommend you to have a look.
Hope this helps!