Originally Posted by
ggggr
Take the mainspring out and try cocking the gun then. If it does not cock or only barely cocks, then it sounds like the piston is worn on the cocking slot and may need welding a little. You can try leaving the cocking lever off and move the piston back with a rod/ screw driver etc. If it engages with the trigger ok then the piston can move far enough back (doing it this way) and that would also sugget the cocking slot.
If the piston engages ok without the spring in, then it sounds like like the mainspring is too long and the gun is coilbound.