Hi Paul
It could be that you have one of the bogey 35's with a knackered cylinder end plug. You can't see this fault but it soaks up lots of power, Rustybuzz posted about this a while ago and I sort of hijacked one of Clomp99's recent threads when I had the same problem.
Fire your gun.
Then the next time you cock it listen at the breach for air escaping just as the piston is raised.
This will be the air trapped in the void in the breach.
I would do one of those linky type things to help you find it but I'm completely computer illiterate and slightly slydexic.
All the best Mick