How do I find out why my dishwasher won't wash the dishes?

My dishwasher does not wash the dishes properly we have checked everything the filters, the spray arms, I put a cleaning solution through it every 3 to 4 weeks but it seems the spray arms are not spraying properly. The model number is ADP8332 Diplomat and is 3 and a half years old. There is water coming in but doesn't seem to do much. I would appreciate any advice.