will it cock with no guide at all ? (wondering if the rod is slightly off centre or binding in the guide ?)
If it won't cock with just piston into block, there is only the rod / profile left... unless of course you muppets cocked the trigger when refitting and forgot to "fire" it before testing
In fact, have you testing cocking and firing the trigger (using a screwdriver to push from near the main rear stud fitting) ? - maybe a trigger issue as Mick suggested above