Okay - so you have established that the cocking rod attached to the piston is too short - that's where the problem lies then!
If you say you can get it to work by pushing the piston back by hand, I presume you mean the rod engages correctly with the trigger mechanism - then that must mean that the spring is becoming 'coil bound' before this can happen when assembled - ie, the spring length is too long for the short length of the piston rod.
if so, you have 2 choices.
1. Shorten the spring - which is what I would do - this will allow you to use the existing short piston rod? Taking some coils off to shorten the spring will sacrifice some power, but these rifles were only supposed to produce 10.5 ft/lbs at their optimum anyway and will shoot probably with 8 ft/lbs + okay - remember, you want consistency over power!
or 2. Replace the piston/rod assembly with a second hand one from another rifle - they can be bought on the bay and other places.
I wouldn't be messing with the piston/rod assembly personally - but if you have the time and skills - go for it.

Does this make sense with any of the othe Airsporter 'guru's that know more about this than I do?

If I'm wrong - then just ignore me!

ASM