My dishwasher isn’t the greatest in the world, but it generally gets plates, cutlery, glasses and metal saucepans clean, so long as food hasn’t been left to dry on for ages.
But for some reason, plastic items just do not get clean at all. For instance, a plastic mixing bowl which had contained tomato-based pasta sauce came out looking like it hadn’t even been near water, despite the sauce residue being fresh, not dried on at all. Washing it in the sink it came off really easily, so why couldn’t the dishwasher handle it?
Location in the dishwasher doesn’t come into it - no matter where I put the plastic things, they don’t get clean. (I know - so why do I still bother? Because it ought to work, dammit!)
The only reason I can think off is the hydrophobic nature of plastic somehow just preventing the water spray doing its job, or something, but frankly I’m stumped.