I have had exactly the same problem with LG dishwasher. After looking into it further there is a mechanism below the bottom spray arm that switches to select whether water is pumped to the top or bottom tray (only one is run at a time.) Throughout the wash cycle the mechanism is not switching to send water to the bottom tray. Even when bottom tray wash is selected, is still running the top tray. Haven't yet go to the solution.
Tony
June 2011