Well: instant diagnosis is that the problem is either the cocking link or the spring.
Firstly check the spring ... do you have a caliper that you can accurately measure the outside diameter with? It does not necessarily have to be accurate, just able to let you see if the new spring is a little bit wider than the original. Is the spring an easy fit into the piston? You might like to try and fit a piston sleeve if there is room.
The cocking link arms may be a little bent and so catching on the spring but my advice is to not touch them. I assume the cocking link can move smoothly in the cylinder and piston when there is no spring there.
Cheers, Phil