Hi,
Helen is right, another thing to try also is this... remove the top basket and on the back wall of your dishwasher is a fixing nut (white plastic) with a black rubber flange. Sometimes this wears (the rubber splits) so the top basket doesnt slide in properly stopping the top arm rotating, awkward to describe but stick your head in the dishwasher and you will see what i mean!
Jason, July 2007