I wonder if the rifle is over sprung. Until you experience this for yourself it is difficult to imagine: a too strong a spring can actually result in a decrease in power. Certainly the amount of preload you have ... and I assume by preload you are referring to the length of spring sticking out of the cylinder before you try to put the trigger block back in, is quite a lot. My 77k runs at about 1cm and it is no effort to fit the back block.
Can you try removing all preload (power) washers and see where you get to? This may tell you a lot.
Cheers, Phil