Could be it doesn't like the rest. Almost looks like it's stringing vertically and to the right, although I don't know the order of shots. I would try was bodiej suggested and change the rest. Plus, of course, another type of pellet or two. In fact, try some different pellets first. Might clear it up and avoid the sleuthing. If external variables like screws, crowns, hold don't do it, then I guess you will need to look inside at the seal, etc. Good luck. Nothing worse than a gun that won't hold POI.