Could even be the spring thats a tad too long,if its cocked quick itl catch on the piston rod if its cocked slow it wont engage on the piston rod,ive had a few of these that people have put weights in or new bigger springs and they become spring bound,but some do cock a couple of times then wont,dont give up on it yet buddy after all you or others have done,dont you know anybody that can take a look for you.