sfaw60
This is, without question, an outstanding dishwasher.
It is, also, many decades old. You've gotten some good advice here, so some of this is repetitive. It is, however, how I begin with a vintage 'working' dishwasher I want to restore.
1) Clean everything, especially the filter. KitchenAid relied on filters much more than did GE or Whirlpool.
2) Check that the rinse-aid and detergent dispensers are actually working.
3) Water level is critical in these machines. Right below the 4-arms. Or just barely not touching.
4) Speaking of which, are the holes all clean? Is the support broken (easy fix)? Are they rotating during wash?
5) Water input has to be at least 140F, we use 145 for our KitchenAid 15 Portable. Run the water at the tap until it really is where it needs to be. Anything less is going to diminish cleaning.
6) Only the best detergents will do. A teaspoon of TSP (the real thing, not the substitute) in with the tab or powder will make a gigantic difference.
7) Ditto rinse-aid. Lemony Shine seems to make all the difference in the world.
8) Loading. This is the one and only aspect of KitchenAid which I dislike. You must load things PRECISELY according to the manufacturer's directions or some things in the top rack won't come clean. The constant spray is nice, but it (obviously) can't clean the underside of things in the top rack, should the lower arm not reach them.
9) Is everything draining properly? Any siphoning? Either can cause the problems you're having. A partially blocked drain, especially.
Lovely machines. Wish they'd never been sold to Whirlpool.