Does it punch clean round holes in the target card? After each shot, does the loading bolt stay in place, or does it move backwards?

Mine did the latter which may have led to a little loss of gas, but it didn't shoot low like yours. I added some pressure on the detent ball by stretching the spring which is under the rear sight. It is moving 7.0gr pellets at just under 500fps, and is bl00dy loud!

Have you checked the barrel crown for any burrs that could be deflecting the pellets?