It's not the retracting of the striker that does it. It's the striker coming forward that moves the bolt forward and that's the darnblasted thing that's chopping the balls.
This could be caused by a number of reasons, a irregular feed of balls is just an example. The best thing to do is get someone with a bit of knowledge to have a look at it, he should be able to tell you what's up, and suggest the appropriate course of action.
It's pretty hard to say what's wrong with a marker without having a good look at it.