The Vader ROTJ is made without a real Graflex clamp--a real clamp has tabs to help lock it in place (in addition to clamping :lol)
Most replicas use some sort of cut down pipe--if it's not lined up and secured very well, I suppose it could sag. Especially if the builder doesn't put something in that area to help secure the clamp & control box in place (wood, PVC or whatever)