In my experience, plastic reels need to be clean and perfectly dry. If they are not, the film will stick and buckle as you try to load it on the reel.

Also, check the little ball bearings under the lips. If they are sticky, they can cause problems. Burrs on the edges of the reels can cause problems, too.

Other than that, check to see that your film isn't curled too much and that it goes on straight and true. This can be a problem with 120 film because of the width between the reel flanges.