If you want to take a set of images into Camera Raw photo merge and you happen to have used Perspective on your last time there.... guess what? Chances are Camera Raw will NOT merge them! Perspective is only available or works on certain groups of images and most of the time your particular set of images will just not work there!
This is a big big mistake on Adobe's part. No where do they explain this scenario. So smart sometimes yet can be so F'ing dumb in others! It only twigged with me after really thinking about what I had done the last time. This merging should go back to default and not what was ticked the last time.