Our Jenn-Air dishwasher has not been cleaning well. There is residue on the dishes, especially the top rack items. There is also a grayish/black build up inside the dishwasher. We use Cascade detergent. When we purchased this model the technician stated that we didn't need to clean any filter since it comes with a built-in disposal. We are wondering what is causing this problem and how to go about fixing it.