I tried to mount my 500mm Hasselblad CF lens to my 503CX camera today and much to my surprise it would not register or click into place.. Yes, both the camera and the lens were correctly cocked and the red marks were aligned,. The lens would rotate but would not mount. I tried other lenses on the camera and all mounted correctly. I also tried the 500mm with another camera and no go, although it also worked perfectly with other lenses. Can't imagine what the problem could be. Please help.