Poolsean is right on the money. I would guess that you have been backwashing the DE filter and have not broken it down to clean it and the DE has bridged the laterals. Tearing down the filter and cleaning it would be the first step as Poolsean suggested. If this solved your problem then I would suggest breaking down the filter and cleaning it about every 3 months (your milage might vary) instead of just backwashing all the time.