[I'm confused]

So I know this isn't the best spring powered rifle out there and I bought it years ago(since upgraded to a src36). Well I lent it to my brother for a year and when I got it back it was slam firing. He said he took it apart and couldn't figure out why. So I opened er up after reading quite a bit on forums and checking out YouTube. The sear looks fine, I even bought a new one and compared them. The piston is plastic but show no real sign of wear. The only thing I noticed is that the bolt carrier has been torqued upwards slightly. As in when you lift the bolt handle to cock it back, someone has pushed it too far up. It doesn't seem to affect it all that much but I really dont know. Does anybody know what else it could be?