Is there a possibility that your ISO was set higher on the 1D III, and maybe you were shooting manually (with either the flash or the camera) and your shutterspeed was too fast to sync with the flash? I know it sounds silly, but I've done something very similar before.


Try a controlled test--set your camera(s) to shutter priority 1/160 sec with the flash on camera and see if the images come out dark. If they do, the flash might be trash-worthy.