I understand your problem completely. I have the same problem with some of my older engines. The problem is most likely your spinner back plate is plastic. The plastic back plate is softer plastic than your prop and there for can not "bite" into the prop. There are a lot of ways you can fix this. Some are controversial, some are expensive.
-- The first thing I would try is to clean the spinner back plate and prop with denatured alcohol. (you can get at the auto supply store for cheep) The slipping may be caused by the oil in the fuel.

OR take and make a washer from sandpaper. If you use sandpaper with the sticky back you can fold the sandpaper in half so the two sticky sides come in contact. This will give you grit on both sides of the paper. Cut the paper into a round shape, put a hole in the center. Place the sand paper between you spinner back plate and your prop. --

OR you can turn the engine so the piston is about 2/3 the way up with the glow plug out. Fill the engine with glow fuel through the glow plug hole. then replace the glow plug. This will hold the engine from turning while you tighten the prop. once the prop is on, take the glow plug out, drain the fuel, flip the engine with your starter for several seconds to dry out the piston. then replace the glow plug and your done. This may fowl your glow plug, so if you have one that is no good, us it to hole the presser then put your good one back in before you fly. --

OR you can get an aluminum spinner and back plate with the hash marks in the spinner back plate to bite your prop.

I hope one of the ideas will be helpful.