seek and you may find...
The major problems with these washarm supports was that the little white bushing inside the outlet which can be seen in the pictures would come loose and move up to block almost all the water from going into the lower washarm thus making it not turn as well as not cleaning the dishes very well. The top arm would then get all of the water pumped up to it. I know people who would just complain that the dishes just were not as clean as usual but the top rack items were perfect.
Check to see if that is the case here and I would also strongly suggest that you take the wash arm support apart and see if by some remote chance that the fine mesh filter has not worked its way thru the pump and in to pump channels and is preventing the water from coming out of the arm.
The lack of the fine filter basically negates the collection of soil in the soil chamber The filter would trap the particles ground up by the disposer assembly in the pump and when the motor reversed to drain, the soil would be flushed clean and out of the chamber. So now, you have all that soil being recirculated right back into the incoming water stream and redistributed on to the dishes.
The filter mesh was never offered as a part and you would have to replace the whole support which is now NLA. But you can find very fine mesh at McMaster Carr or maybe a good hardware store. You can remove the small screws you see in the picture and fit the mesh on to the frame and reattach everything and have something that will outlast the original mesh. Just make sure that is made from non russting material and can withstand high heat.
As mentioned above, you may get lucky on ebay.
I for one will not part with my spares just yet as I have a 21 and 22 machine as spares and would not want to be caught short wthout the extra supports I have at this time.