I would go straight for the DE & sprinkle some on her back, nests, roosts & general coop chicken hangout areas (or maybe a mix of DE & pyrethrin powder), if it doesn't fix soon then go for the above mite remedies that would surely be needed to eradicate this issue. Maybe that's just the stubborn organic girl in me to do the lesser first. Also, if you have a rooster then he may be breeding her more often. But, are those black specks seen on her back in your pics mites or dirt specks from dust bathing? She sounds like a fine hen, who gives it her all. Best wishes.