RB67 w/ProS Bck, will not advance...

This morning, took out the RB67 with a ProS back, and shot number #3 vertically, and then advanced film, and changed the exposure time, took a shot, everything working fine, but could not advance the film.
Noticed that it was like the film had not been shot, the little red flag next to the number was not there.
Hmmm,,, ok lets try again, made sure the dark slide was all the way out, camera fires, still cannot advance film, still no red flag. Hmmm.. tried again... no luck... Took back off, nothing seems to be wrong, put back on, again try #4.. at last red flag.

To make a long story short, this back has never given me any trouble, but kept on doing this through #6, when I just finally screwed on the lens cap, turned the back Horizontal, and fired it twice, before the red flag would appear. Changed the lens settings, made no difference. Finished the roll by taking a shot then screwing the lens cap back on, cocking it, and firing it twice before it would allow me to advance the film.
What's going on?
Have not developed the film yet.. going to have a lot of multi exposures, but ProS back is empty now, so what should I check, and how to do it?
Thanks in advance.

Check that you have fully rotated the back into position.

The backs and the rotating adapter have seals that can degrade.

Backs and the the rotating adapters sometimes need CLAs too.

Its been 12+ years since I worked on a RB67. I just did a search for the service manual, this was the first result: http://rb67.helluin.org/files/2010/02/Mamiya-RB67-Pro-S-Repair-Manual.pdf
Page 25 covers your problem. It can be the body as well as the back.

Thank you both but investigating further with the camera, I'm taking this to Camera building and Repairs.

@
(there was a url link here which no longer exists)