To make completely sure, develop a piece of film that's not on a reel, but cut a 4" strip of film off and develop that in your tank or in a tray (total darkness of course).
How do you load your film? In a changing bag? In the darkroom? Is there anything in there that could scratch the film? The advice on squegees is a good one. My reels are the plastic kind with the really large catch for the film strip; it covers all of the width of a 35mm film as it passes through onto the reel. If that's what you use, something might get caught there to scratch your film base. Doesn't take much. It has to be in a stage of the process where the film is held steady, if it's consitently at an equal length from the edge of the film. Those stages are:
1. Loading film into camera (not for 120 film, since it has paper backing)
2. Loading film onto developing reels
3. Squegeeing film after development and rinse
That's all I can think of.
- Thomas