Dirty dishes. I have a zanussi slimline dishwasher, but no matter what I do the dishes still come

Question

I have a zanussi slimline dishwasher, but no matter what I do the dishes still come out gritty and sometimes food particles stuck to inside of cups and glasses, glasses have a white film over them and have to be done by hand, pointless really, I use finish tabs or persil or anything that is of good quality, and use a rinse aid and salt. What is the problem