I had this problem before. Sometimes the new prop splines are cut so square they won't slide over the shaft splines, or there is a cosmoline coating on them.
Try sliding the prop on backwards to see if it goes on at all that way. If it does, slide it back and forth almost like you're using a file on a piece of metal. This should scuff away any coating or burrs that may be on the splines.
If that doesn't work, take emory paper or sand paper and scuff the splines of your new prop so that it slides on easy. Some vasoline on the shaft splines helps too.
I was njskier on here.