I've had similar issues with mine. It boils down to 3 issues. The first is variation in the cartrige seals. Some are too loose a fit, - some too tight. To overcome this, I made a brass pusher tool,. This tool is just a shouldered brass tapered pin to countersink the pellet in the cartridge. No more pellets left in cartridges and tighter groups!
Then it started happening again, but much worse! This time no gas most of the time, other times the gun would jam in the half cocked position on squeezing the trigger. Apart it came....
First fault was there were still casting flash/burrs on the trigger sear. Also the end wasn't finished square. Half an hr with a swiss file fixed that.
On reassembly I found I had a smooth, light consistent trigger. Only problem was it now dropped into the halfcock position every time.
Apart it came - Again!
After an inspection of the tumbler end of the hammer, I found the halfcock notch to be quite open and the belly of the base of the hammer bulged out, - ensuring the tumbler caught the sear in halfcock position. Turns out some wally had been spinning my gun on their finger in half cock position and also squeezing the trigger with the hammer halfcocked!!
( I was away making some tea)
The remedy was gentle light tapping on the belly with a toffee hammer and a piece of leather to close the sear notch up a little, followed by a little filing of the tumbler belly. Relube and reassemble. The action is now smoother than when it was new, the groups are tighter and the velocity is up.
Sorted!