Our hens have a dust bath and they still get mites. We have a problem with a wooden sleep house which houses them. My husband wand to construct one of metal, iam not happy with this because it is cold. Hens with mites don't lay.
Recently we bought a (pretty expensive) powder at the produce store to mix in their feed as a supplement I think it was diaco... something earth but I am not sure.
We got rid of the mites. And the kids cleaned the chicken house with hot water and eucalypt oil. In former times animal houses were painted with lime against mites and things.
Put some sifted fireplace ashes in a very dry place for them to dust bathe, a good dust bath mix is 10 parts fine dry soil or sand, 1 part diatomaceous earth, and 2 parts wood ash. For immediate mite control, get a big bin of wood ash, go into the coop, and dust each chicken by holding them on their back in your lap and really scrubbing the ashes deep into their feathers. You can put a clean baby sock over their heads to keep their eyes & noses protected from the dust. Do that nightly for a week or so, then check thoroughly for mites.
Check the feathers by their tails as well - feather-eating lice are often comorbid with mites, you'll see the egg clusters on the feathers under the vent, they are cottony masses that look like q-tip heads down by the skin. Cut or pluck those feathers and burn them.
Having a dry dust bath with wood ashes (which alkalinize and dessicate the mites) is very, very important. It should be shallow and big enough for several hens at a time. You can also find the spot they're already dust bathing (I guarantee you there is one) and add ashes to that.