Hope the gunsmith fixes it.
Have you tried individually loading pellets manually without the magazine? If the mags are slightly out of alignment it may result in some damage. The mags may be perfect, but fo they align perfectly?
Worth a try with a pellet like H&N FTTs that usually shoot well with HW100s.

Or you could try shooting into a very deep bucket or barrel of water and retrieving pellets to look for signs of shaving. It does need to be very deep though (probably >24 inches) because otherwise the pellets deform when they hit the bottom.