It is never a good idea to allow backwards movement in quiz slides. And be careful to remain within the quiz scope or the quiz attempt will be considered over. That is probably what happens: if the user is going back and remains within the quiz scope, then the answer will remain, but if he gets out of the quiz scope and you allow multiple attempts on quiz level, answer can have been reset.
Question Question Slides in Captivate - Captivate blog
In this blog post I tried to explain functionality of question slides.