Just to be clear to the OP, the point I was trying to make is - your problem might be overly tight seals in SOME of your cartridges/oversized pellet skirts. A pellet pusher will solve both these problems. Alternatively you might have a subtly bulged halfcock notch in the tumbler, which may be acting as a brake on the trigger sear, slowing the hammer fall, resulting in a light strike on the valve. Rather than grabbing the sear and jamming in the halfcock position. Both are easy fixes.