The last time I saw this happening, the student had failed to ream his propellor.

If you don't ream the hole in the prop, you have to "Thread" it on to the prop shaft. Then you can tighten the nut all you want, but you won't compress the prop hub at all. It will hang up on the threads. The prop must slip over the prop shaft, all the way to the backplate.

If you have reamed the prop, and it slips over the prop shaft all the way to the backplate, please ignore!

Best wishes,
Dave Olson