By jove, I think I found the problem.
Check out this link:
You're looking at the bottom left of the camera back, removed from the camera.
See the tab on the left? That tab prevents the grafmatic from sliding in all the way. The ridge on the grafmatic therefore doesn't slip into the groove on teh camera back, making a light-tight seal. If you do push the back down far enough to seat the notch and groove, the bottom left of the back is raised away from the camera back, again preventing a light tight seal.
My question is this: what the heck is that hole for? It's threaded...
I'm tempted to grind the sucker off.