If the reg isnt teh problem...and it sounds like it isnt, as why would it only do it for one shot?
I would look at teh next option.
In order for the shot to drop off, the rammer has to hit the valve with less force.
So...I suggest that teh problem may lay, with your front QEV or possbly rear.
I say front as it could be acting as a air spring if not venting, thereby slowing teh rammer down.
Just a thought...