I'll bet you are running a low pitch prop. Several of my engines require a spinner back plate for those props or the nut bottoms on the threads before the prop tightens well. The other likely cause is that you don't have a propper front washer and again you are running out of threads. The front washer should be quite thick with a taper on the front surface.

I run into this with the 3 3/4 and 4 pitched props. If I don't have a spinner back plate they won't tighten even enough to turn the engine over. The 6 pitch is usually OK and a corser pitch is always OK.

Don