Why isn't my dishwasher cleaning as well as it used to?
Some detergents will build up over time, gumming up the works. Try changing to a different kind.

Try vinegar. Pour some in to the dishwasher and run a "rinse and hold" cycle. It can clean out scaling from hard water, and detergent goops.

Check that the basin is filling up properly. I read that it's supposed to be just below the heating coil.

Check the water temperature. Let the machine run to fill the basin, then measure. If your hot water pipes are not insulated, insulate them. You can run the kitchen faucet for a minute to bring hot water closer, but it doesn't help much. Consider turning up the water temperature at the water heater. I did this and my dishes came out much cleaner.