👩‍🎓 I set my student a watch assignment. They told me they'd watched it but it's showing 0% in the assignment.

If students don't access the video through the assignment, it doesn't count in the assignment.

It's extremely likely that the reason for this is that the student has chosen to access the video through the course, rather than through the assignment.

If this is the case, they will have seen this message:

Clicking on "Register my activity in the assignment" would have taken the student to the tutorial inside the assignment. But if the student clicks on "Ignore the assignment and continue", they will be able to watch the tutorial but it won't count towards the assignment, since this is what they've chosen to do.
It is a good idea to mention this to your students and make sure that they click on "Register my activity in the assignment" next time.