Jesus fuck, you wonder why everything smells like mildew, it's because they ball up in the wash and don't rinse properly, and then they stay balled up and damp in the dryer.
Do you wash only a single fitted sheet or duvet cover at a time? Because the problem still happens when I wash my fitted sheet and normal sheets in the same load.
That can happen, but I find that if you separate the sheets as they are going in, and kind of scrunch up the fitted sheets and duvet covers so they don't start wrapped around the flat sheets or blankets, then they are far less likely to ball up in the wash.
I had a roommate in college that would wrap his sheets in the fitted sheet (along with other laundry) and throw it all in like a package. His laundry always smelled like mildew.
Or just get some laundry bags. Its honestly so much easier to just shove the offending item in a bag, throw it all in the wash, then I don't have to care about it until I take it out of the dryer. Come in handy for other items that love to ball up and evade washing.
I pick them out of the machine and put them into a dryer or set them out to dry. Seems to avoid part of this particular problem.
If we're out here handing intructions, you preferably shouldn't uses a dryer for your stuff anyway. It can damage them. Especially everything that's supposed to be elastic.
But then again, I don't always follow that rule. Some stuff with an elastic band I've put in a scorching hot dryer for close to a decade and it's fine. Some garbage shit lose the elasticity after a year or so even though you've washed them at 30 and don't ever put them into a dryer.