I run a Moodle plattform and I came across this very puzzling problem:
A Course Presentation one of my authors designed was shown in the Moodle course, but when you click on it, it is not displayed on the next page, that is empty. If you want to work on it, that's possible, but the results cannot be shown.
After some very extensive fiddling, I hit on the fact that "Single Choice Set", "Drag and Drop" and "Drag Text" cause the problem. When I remove them, it runs just fine.