sounds like the gun is re-cocking before it has had chance to fire
ie. the cocking rod is being moved backwards by the ram before the hammer has had a chance to hit the valve
is the back block tight against the back of the body (there should be about a 1mm gap) ?
you could try to manually cock the marker and then fire it without the soleniod connected to prove this. without the soleniod connected it should still release the sear but not activate the ram