I have a chicken...Barred Rock....with a significant amount of feathers missing from her back. The other hens are OK. I have tried minerals. I have tried seperating her from the others, while there may be short term improvements, nothing is working for the long term.
"Good decisions come from experience. Experience comes from bad decisions." ... Mark Twain
If you have a rooster, she's likely that rooster's favorite.
When a rooster mounts a hen, roosters sink their claws into the back of the hen. Repeated mountings of a single hen ends up pulling a substantial amount of feathers out, extending all the way up the back.
I'm not aware of anything you can do about that.
I have one rooster, 22 hens. But he doesn't spread his "affection" evenly amongst them - in my last three flocks, there have always been one or two hens who got it worst.
Maybe if you had a dancing rooster, it'd help? I don't know, I never had a dancer.
(Roosters who dance before a hen are asking consent before mounting - maybe then the hen could decline to be mounted if she's overused?)
You could seperate her for awhile so she could recover, but they are flock animals and being separated might bother her.
You could cull and eat her if it gets too bad, and then someone else would become the favorite but would likely be fine for a few months.
You could cull the rooster, eat him, and then all the hens could have a break while you grow new rooster.
I don't know of any way to make a rooster rotate hens in a more equitable manner.
It could be over-mating, if you have roosters with your hens. But it also could be mites or lice.
“When the last tree is cut, the last fish is caught, and the last river is polluted; when to breathe the air is sickening, you will realize, too late, that wealth is not in bank accounts and that you can’t eat money.” -Alanis Obomsawin
I have 21 hens and one rooster, at least 10 of my hens have no feathers on their backs, they do not have mites or other parasites. And the hens do not need to be forced by the rooster. In my flock the hens usually get down in front of the rooster when they are willing. Happens pretty regular, Its natural.
Be Content. And work for more time, not money. Money is inconsequential.