Water delivery issue!
The primary reason for poor washing results is usually a water delivery problem. That is, there is either too little water in the tank to pump properly or the wash arm support is worn out and the wash arm is not getting the proper amount of water through it to revolve and spray the dishes.
The wash arm supports on the 18 thru 20 series machines and later, the 21-22 series were notorious for the small white bushing inside the housing loosening up and moving up to block the water going into the lower wash arm. Sometimes the dishes would come clean just due to the sheer volume of water being sprayed out of the top arm but you really need both of them working in tandem.
Check the water level in the tank when it completes a fill and it should be within about a 1/2" of the bottom of the lower wash arm. If not, then you may have a clogged strainer in the water valve or just poor water pressure. At this point in the age of the machine, it would be best to just replace the valve and be done with it.
I would start with these items assuming the pump sounds good. Once in a while, the wash impeller will wear down or lose a vane and that may cause a loss in pressure as well. This is a remote possibility on your pump as it had the closed vane impeller when these were produced.
Again, check the water level and the wash arm support and go from there. Also, be aware that if there is not enough water in the tank, the detergent door(s) may not activate as the bimetallic springs that release them depend on a full charge of water in the tank to draw enough current thru them to bend out of the way and release the door(s)
Hope this helps.
