DE does nothing for a mite infestation...you need permethrin or Sevin dust to treat mites...
All of the chickens will need to be treated as well as the coop...burn all bedding and any bedding for nesting boxes too...I do not know if you use straw for nesting boxes or bedding...if so replace with something else...straw can harbour mites...
I suggest that as you said had evidence of mites that this be your first priority...mites will suck blood and kill chickens...
Sorry if this sounds dramatic but it could be the reason for her being ill...
All of the chickens will need to be treated as well as the coop...burn all bedding and any bedding for nesting boxes too...I do not know if you use straw for nesting boxes or bedding...if so replace with something else...straw can harbour mites...
I suggest that as you said had evidence of mites that this be your first priority...mites will suck blood and kill chickens...
Sorry if this sounds dramatic but it could be the reason for her being ill...