And I'm betting you're not getting the prop tight enough. You really have to clamp down on it, and don't be afraid of tightening it, it's pretty difficult to do damage, although you CAN over-compress a wood prop.

If you have a spinner backplate, then there's no way your prop nut is bottoming out. (Running out of threads.)

Tighten it down real good, then tighten it some more.
Dennis-