Possible conditions:

attempting to start in an overprimed condition with possibly even a hydro lock, what position is the engine mounted in?

Still not tightening nut enough. What exactly are you using a spinner nut, regular nut or a spinner of some sort?

Agine did you ream the shaft hole or does it go on tight over the threads? If you force it over past the shafts thread shoulder then this causes an inability to tighten the prop enough. You cannot use the nut to try and force it all the way up to the thrust bushing knurls.

You won't be able to get an objective answer untill you come up with some answers.

John