Suzie is quite right; I missed that. They need an effective treatment. Your hen's symptoms may very well be strictly from the mites.
http://edis.ifas.ufl.edu/ig140
http://anrcatalog.ucdavis.edu/pdf/8162.pdf
i also agree that wormind would probably be something to consider seriously. If...
One possibility is that this chicken is weak due to an underlying illness which is impeding her recovery from the mites. Did the others get pale combs from the mites? A pale comb isn't getting enough blood, which can have a lot of causes. Maybe the bird is anemic, maybe in heart or other...