Searched around and saw this problem a lot with the lens stuck on the body, but my situation is slightly different.
I just got in my RB67 body today w/ film back. Cocked the lever went down all the way shot it and it worked nicely. Did the same thing right after for a second time and when i put the lever down it came back up half way. Now its just sitting there and wont let me fire it theres a little play to move down. So i was thinking maybe i didnt take it down the lever all the way. that wasnt it. Tried a release cable, didnt work. And keep in mind there was no lens on the body when it did this. Maybe i missed something small and forgot to do, and the film back never came with a dark slide. Any help would be greatly appreciated.