Are they free range or locked up? If they are locked up, do they have an area they can dust bathe? I had mites only once. And that was when I don't have a dust bathing area. I put a plastic tub in with sand and dirt mix for them.
Tub is cut like in the pic. I also used a spare one inch pvc pipe for the bottom edge of the cut part, slit one side of pic and put over the edge so the chickens could stand on it. I have 3 of them use it at once sometimes, they don't lay eggs in it because they like their nesting boxes with straw instead of dirt and sand