After wiping down the inside of the lid of my GF it stopped centering. Underneath the print panel was an error message, could not scan from camera.
I had been over enthusiastic with the Simply Green - the glass was very dirty. Some moisture had got in behind the camera. I could see it by pressing the camera cable.
I squirted through with compressed air and let it dry and it all came back fine.
Moral of the story. Spray on a cloth, not on the glass.