Stop using DE it doesn't work as people claim.
I recommend a pyrethrin based poultry dust. I haven't used the spray ever. I use the dust and it kills.
Get a chicken by it's feet with it's chest resting on something, ground or table, and sprinkle the dust on all over. Make sure to get under the wings and on the vent. Best to wear a dust mask. After dusting rub it in with a gloved hand.
Dust all roost cracks and in the nestboxes under the bedding. Repeat weekly for 3-4 weeks.
One treatment will only kill the current adults, you need repeated treatments to kill the hatching eggs. Mites will live in cracks in your coop. Wild birds can re-infest your flock.
A good dust bath area will help the chickens keep control of their own external parasites. A few mites can be normal.