I recently purchased a Mamiya rb67 pro-sd body in excellent condition, a 90mm sekor C lens, waistlevel view finder, Pro-S revolving back adapter, and an a rb 120 back. The revolving back adapter will only allow the shutter to be released in the vertical orientation. When I rotate the back to the horizontal position, it locks into place with a click. However the shutter release button will only depress about 80% of the way and will not release the shutter. Is there something I'm missing? and yes the dark slide was removed, shutter was cocked, and film advanced. I've tried everything I can think of including trying to test the shutter without the film back. no luck any help would be greatly appreciated. I'd like to avoid buying another revolving back or even returning this one.