Mites are the size of a pin head, really small, she sounds really poorly, the only other thing it could be if is worms, have you wormed your chickens, - if it it mites or lice, I suggest you use Frontline, it what they use for cats, but its the only thing that has worked for me, but if it is...
Have you checked for mites/Lice...this is how two of ours died a few weeks ago, symptoms sound exactly the same...is she underweight and lethargic ? - sounds similar ...