I just got an RB67 in from KEH. I've looked on the internet, looked at the manual and the shutter cocking lever will not bring the mirror up. It seems to only move a slight amount, not fully down as it should be. The mirror does not go up. I've tried it with the back and lens on, I"ve tried it with them off. What am I doing wrong???? Is there something that locks up the mirror??? Thank you in advance!