There was a mysterious malfunction preventing people from moving from lesson 16 to 17 for the Gelio Edhellen! 100 course, and I finally figured out the problem!
There was an option that sounded great on the quiz-set-up that I selected: “Do not store user answers in the database to save DB space.”
“I want to save database space!” thought I, clicking it.
Turns out that this means it doesn’t save ANYTHING, not even that someone took and passed a quiz. That’s how I figured it out, actually. Quizzes I had screen-capture proof of being completed were marked as “taken 0 times.”
Sorry it took me a month to figure this out! We can now go back to our regular schedule of new quizzes!