You answered your own question in your 2nd paragraph.
If it is moving back far enough to eject and feed the next round , it has gone way past the cocking of the hammer phase. Your hammer is not being held by the top hook and is following the bolt back . Very bad Mojo here. Good thing your reciever is cut right or you'd go full auto , I'd think. When you re-cock it , I bet you let the trigger go , and you're catching the hammer on the lower hook. With it empty and the trigger held back , work the bolt and see what the hammer does.
I'm guessing trigger is bad or he filed on the hammer.
Chris