I'm not familiar with PDWs either, but here's some general troubleshooting:
Do you still have any green gas mags? You said you "switched", so I don't know if you traded them all away or something. If you do, check to see if the green gas mags are still efficient. If they are no longer efficient, then there might be a problem with your gun. Eg a cracked nozzle is letting all the gas escape, or something.