I am new to this group and forum. I have always found answers on apug when googling problems. I decided to join since this seems to be the place to get reasonable and knowledgeable information. I have a Mamiya RB67 ProS. I recently got a ProSD back and have been using it. I really like the dark slide pocket. Everything seemed to be working fine but in the last week, every roll but one that I've developed has had one blank photo. The last roll had two in a row. I ruled out the lens by switching it. I took the back off and cocked and fired the camera about 20 times in a row. The shutter on the lens worked and the mirror in the camera worked as well. Could it be the back? What would cause a problem like this? I don't look at the counter every time I cock the film advance but when I do look, it seems to be working. Shows red for exposed and won't let me take another photo until I advance. Thanks for any light you can shed on this.