Thanks for the suggestions. Unfortunately, I still can't get it to work! Incredibly frustrating. I've disassembled and re-assembled the bolt at least a dozen times now and it's still not working.
I've noticed that when I put the bolt in the gun, it is fine until I press the bolt down to lock the action. A couple of times it has released from this position on the first time or two that I try it, then it locks in and I have to disassemble the whole bolt to get it out of the receiver.
My suspicion is that this does have to do with the cocking button, although my problem is somehow when the bolt is in the receiver.