huh, I actually had a similar problem last night when I was screwing around with my trigger box. If you hold the box so you are looking at the right hand side of it, there's that little plate that screws onto the side with the 2 screws. Take that off. Underneath that is this little sort of U shaped piece with a spring in it. I didn't put it in properly the first time and it gave me the exact same problem as you.
Take the piece out. Make sure the hammer is forward. Then put it back in carefully and make sure it moves smoothly all the way up and down in its groove. Then put the rest of it back together and see if that fixed it.
I noticed if you try to put that piece in with the hammer back, it wont go in quite right and it wont move down all the way.
edit: this is the u shaped piece I'm talking about
http://www.airsoftparts.ca/store2/in...oducts_id=1782