Problems with a button's state working

I'm having inconsistent problems with a button's state. I currently have a course that has 5 content sections and 1 quiz section. The user can click on any of the 5 sections in any order they want, and once all 5 sections have been "visited" the quiz button becomes "Normal". 

My problem is that a few users have recently not been able to access the quiz section (button not changing to normal) even though all 5 sections have been visited. I have taken a screen shot below.

There must be a reason why in some cases this happens, but i'm not sure what the problem is. I have also attached the course to this discussion. I hope someone can help?!

3 Replies