BDW13 does not rinse on bottom tray?

MY BDW13 dishwasher does not rinse properly on the bottom tray, though it works fine on the top. The lower arm doesn't spin even though it is free and there doesn't appear to be any water coming out of it. Is there a filter somewhere that needs unblocking? I have cleaned the main filter at the bottom of the dishwasher but still no better.

Rick Booth
September 2015