hoover crystaljet D820

I have a Hoover D820 Crystaljet dishwasher that is making dishes on the
upper left hand side of the basket dirtier than when loaded (even plates that are relatively clean prior to washing have bits of food from other plates on them after the programme has gone through).The filter is always empitied & both upper & lower spray arms have been replaced without success. Cups etc in the upper right basket are sometimes left with a gritty residue but the items in the lower basket are always sparkling clean, although nothing seems to get dried now. We have tried different ways of stacking with no success. Any thoughts please?