Buttons disappear

I have another problem that some buttons disappear when a certain button has been clicked. Can someone please have a look at the fla I attached and help.

As soon as you click on the button “knowledge types and processes” and navigate through the arrows in there til the last page and then click on quiz for example, the button disappears as well as every other button you click on afterwards.
There is hardly any actionscript. Most of it is just jumping from one frame to another (gotoandstop()).
I don’t get it but I really need to pass this course.

Appreciate your help,
Franni