Hi there VacuumGuru here. The only thing that can really stop it turning is either an obstruction or your brush bearings being siezed. It is a little difficult to describe the steps to solve htis problem so i have made a video. It shows how to check all the way from the the obvious right back to a faulty clutch. Have a look at the video and if it is helpful please take the time to give me a thumbs up or a testimonial. If you find that you need parts for your dyson then check out vacuumspot or nelsonvacuums.
Hope this helps
Guru :)

Click here for video