[MadMax]

Wow that helps a lot. It looks like a copy of the TM Hicapa blowback design. I guess it's a copy of their Glock.

Out of curiosity, could you try reassembling the whole thing without the spring?

I'm guessing that the valve reed is not popping forward and closing the opening to the breech so the gun never goes into the recoil mode. Without that spring it should go into recoil mode without pushing out a pellet because the reed pops forward immediately instead of when the barrel pressure drops when the pellet leaves the barrel.