If the plates don't clean at all sometimes, I wonder if there is actually any water coming into the dishwasher? Is the detergent tablet dissolved or simply lying in the bottom of the cabinet? if so, I guess that its the inlet water solenoid that has failed - its simply a valve that opens with electricity to let water in. If it sticks shut, then no cleaning! A simple and relatively cheap fix, usually.
John, April 2009