Kenwood dishwasher who's rotating arm falls off?

My kenwood dishwashers upper rotating arm spins off regularly in the wash cycle. I cannot seem to find a way to secure it . It has a small plastic disc which clips into the screw on bit, but you can see it won't take much for it to come apart. You can sort of Tighten it by turning it when in place but not enough to really secure it. It will stay on sometimes for 3 or 4 washes and then for no apparent reason will come off every time . It's driving me Mad! I'm thinking of glueing it in place. Anyone had the same thing?