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

Mike Dixon, October 2009

Hi Mike I think this model has a mechanical diverter to alternate between upper and lower spray arms, so it could be a faulty diverter causing your problem.

Mike, October 2009