1) Mistimed: You can fix this with resetting it to factory settings.
2) Lug is too far down/sear not dropping properly
Take out the bolt and put an allen key down into the top hole in ur body, (make sure its the right size) when the marker is decocked.
You shouldnt need to do this if u can cock the marker manually and fire it with no gas in it. cock, fire with no gas, and see if the cocking rod slams forward
3) Valve broke:
Send to planet, or me, or belsales
4) Velocity uber low:
Turn it up duhh
5) inline reg not set properly.
Search for sweet spotting regs.