Hide/Show quiz based on content completion

Hi there, Natalie! Your experimentation with the restricted navigation was the first thing I was going to recommend. 

Once completed, your LMS may be treating visits to the course as "new" attempts, and is discarding the data that lets learners keep their progress. Until the course is completed, however, restricted navigation will allow your learners to revisit previously completed lessons. I hope that's what you've seen!

The only other alternative I would offer is to create your quiz in Storyline, and restrict it with a continue block in the same lesson. Here's more information about tracking Storyline blocks for completion!