The easiest problem would be if the rotating arm was prevented from rotating. The top rack of dishes is then not washed. You have to actually remove the motor from the bottom to finish the dis-assembly of the whole pump system. I had to do this for a leak in the pump system. This really isn’t that hard to do.
My dishwasher had a similar problem of low water flow and less than clean glasses on the top rack.
I unscrewed the grate at the bottom, removed years-worth of popcorn shells and plastic wrappers, and ran a cycle with bleach to cut through the slime.