DE is nice in their dust bath but won't help with the mites.
Heating up your coop to at least 60-70 ° Celsius for the duration of 1,5 - 2 hours will kill them. And if you still find that some come crawling back from the outer roof, just use a blow torch or hot air gun to kill them.
You can also...
Is she on a clutch of eggs?
In any case she should be taken off the nest daily for about 15 minutes to defecate, eat, drink, dustbathe and preen.
While you pick her up, inspect her closely for lice and poultry mites as they can suck the life out of a broody hen within a few days
Adds up to 5...
Do you have a rooster with them?
Take a closer at their necks. Do you see pin feathers coming in? If so, the movements you see are their attempt to escape the hurt these pin feathers cause them when they move their heads/necks to certain positions.
While the adult hen could be moulting, the...