I'd say you've probably got a contact issue somewhere. I've seen what you're talking about and both times I had that problem I had a contact issue that I needed to fix. What kind of rest and vanes are you using??? Are any of your vanes notably marked up like you're getting contact with something??? Also, have you had someone else what you shoot to make sure that it really is doing what you think it is??? Sorry to answer your question with more questions, but that's the only way to know for sure what the true "root" of the problem is often times.