RB67 Pro with Pro S back - shutter problems

Hello, I'm new to this forum and am hoping someone can help out with a RB67 I recently bought. It is the RB67 Professional body but has a Pro S film back on it. It seems to be in very good condition and working well (although I haven't put a film through it yet) but there seems to be a problem with the rotating back. In horizontal orientation the shutter release works fine, but when the back is turned to the vertical position the shutter release doesn't work. There is no click as it is turned to the vertical position, but, the shutter release does work in the vertical position when the film back isn't on. I realise that having the Pro S rotating back might be part of the problem, but I can't seem to find anything online where people are talking about this specific problem. Any help would be much appreciated!
Thanks
Greg