Let's say if that will be the case, it should happen for 1 roll of film not 3 or 4, and i said for first roll ever i've got 3 shots out of 10 and all other were blank and the lens cap was off, so it is something i really don't know, i have a big feeling it was the lens, but the seller on ebay told me that as long i've got 3 shots then it is not the lens issue, and now you said that the battery is not the issue, in fact i can't advance to the next frame until i open the back and do something i forgot, but i couldn't when the back is closed.
The only things i can do now is waiting to buy new fresh batteries for my Mamiya RZ and this Mamiya 7II and give it a try, also to buy another lens to see if i have the same issue as well, returning the lens or the camera is not my solution, and it passed 2 years now since i bought them both separately.
Wait, you opened the back in order to advance the film? so you exposed the roll... no wonder you have blank film LOL Unless I misunderstood you?
If it is in fact the lens, the seller should take it back or file a claim with paypal/ebay for it.
~Stone | "...of course, that's just my opinion. I could be wrong."~Dennis Miller