I had the very same problem with my Saito 100 and a plastic spinner back plate. The solution for me was to take some very course sand paper and rough the back of the prop up, sand from the center out in a straight line not a circle, and then clean all the oil of the prop and back plate. I must have spent an hour getting that prop to tighten! For what ever reason a Master Air Screw prop would not tighten, I had to use a APC. Make sure you use a good solvent to clean the mating surface of the prop and back plate after roughing up with the sand paper.