To make sure the wash arm is perfectly clear remove it by pulling it up out of its socket then carefully remove the spout from the arm, its clipped on , then repeatedly tap and shake the arm at an angle so the debris etc falls out of the central hole. If this is clear but does not make any difference then the fault is most likely the diverter valve is jammed ( the water only comes out of one arm at a time if you open the door you will only see one arm spinning but it should alternate from top to bottom repeatedly)If it only operates one then most likely diverter is faulty total strip down to replace this part not diy project.Cheers
Kenjo, June 2012
