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Well, some of our pans - stainless-steel roasting pans, for example - always need a soak and then a scrub /scour before going in the dishwasher. No, mugs and plates just need a rinse... but I hear tales of people just bunging in all their utensils straight off the stove/table.
All I can think is that their filters must be regularly bunged-up with detritus, and Mrs B says on the very few occasions when we've forgotten to clean properly first, we end up with grease on the dishwasher walls, too.
BB