My first question would have been "did you change foods?" so that is no doubt where your problems lie. Even the BEST brand of food is not right for EVERY cat. It needs to be the perfect combination of ingredients for YOUR cat. Admittedly, starting with a premium food increases your chances of it being the right food, but just don't be surprised if it helps one cat but not another.

I feed a presciption diet (Hill's C/D) Now, I do not suggest everyone change to that but I can tell you first hand the difference it made in the coat of one cat in particular. Y.Lee. When he came to me he had the typical dull and flaky coat. The type that even though it was short hair, it would mat. I started him on the C/D and miraculously he became shiny and silky. No dandruff and no tangles. Later when he was diagnosed with diabetes he needed a food change. One with less fat. His coat suffered terribly. but it was what he needed to try to keep him healthy.

the moral.....it is OFTEN food related.

By the way, all my cats have marvelous coats due to the C/D.