[Jimski]

I guess you have the stock spring -

1.make sure your battery is fully charged and working
if you are unsure, discharge/charge it fully again.
check it with a multimeter.

2.check the shimming, the anti-reversal latch position, when you close the gearbox, check that
-the piston snaps in its rails
check the shimming
-the trigger system is properly mounted
check the shimming
-the nozzle is held by the tappet plate
check the shimming
-your underwear is clean
check the shimming

dry fire it with putting LIGHTLY your finger on the nozzle to retain the air so the piston head doesn't slam violently in the cylinder head.
check the shimming
(don't 'seal' it completely with your finger)
pray.


check the shimming again.