Mamiya RB 67 ProSD back problem?

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.

I ruled out the lens by switching it.

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.

Is the unexposed frame in the same position in the roll? Any similarities or differences between the rolls?

I bought a 180mm K/L at auction that I ended up returning because the shutter would occasionally not fire when using the camera (Pro SD) in T with both mirror lockup and bulb. In this case it had nothing to do with the back.

Regards,
Rob