My Neff S5555 Dishwasher Spray arms work intermittently?

The lower spray arm is currently not working, yet sometimes it is the upper arm that does not operate. I have cleaned the filter and the pump area, and also cleaned both spray arms.
There appears to be sufficient water pressure, but only one arm at a time rotates.
I have removed the grey housing into which the lower spray arm fits and there does not seem to be any blockages.The dishwasher is five years old and is used three/four times a weeks