Users skip questions and passes quiz

We've found that a few users are not answering all quiz questions, yet they pass the quiz with a score less than 100%.  The reason they're passing the quiz w/out answering all questions is, we believe, that they're not using the "next" button; rather, they skip through the material via the Table of Contents.

Our TOC menu options are set to "Free" navigation restriction. If we set to restricted or locked, will the user be able to navigate at will through the TOC after they pass the quiz? We've noticed that by setting to "restricted" navigation from slide-to-slide is much slower.  Is there a minimum number of seconds built into this setting?

Any feedback is greatly appreciated.  Thank you!