No easy answer here, you will have to make sure all the hoses and spray arms are clean and clear. Make sure the dishwasher is getting enough water. It should fill until the water level float shuts the water off, if it shuts off early the water valve may be bad and not letting enough water in, therefore you get bad wash because there is not enough water for the pump to make good pressure.

If all that check out you may have to open up the pump and inspect the impellers for damage or to see if anything in there is clogged (filter body-waterways etc)

Good luck and let us know what you find so we can get you the correct part.