Agree you either have a feather eater in your flock that's picking on her OR some form of parasites. Didn't Catch if this was an adult bird or not. To be safe 're the parasites, I'd Consider
Treating ALL your birds because if one has bugs, they can infect others. Here's what I'd do:ck all of your birds for tannish gritty looking deposits around the vent and also around base of feathers near head. Fan out wings and look for the same. I spray CAREFULLY with a small amt of Adams flea and tick spray (for dogs and puppies). You can get this at any pet/livestk supplier. CAREFULLY not to spray near eyes or directly in vent. Use a cotton ball sprayed with it so you have more controll. So swab bird around vent, under wings and back of head. Do this at night when bugs are more active apparently. Also, if she does have these gritty sand-like clumps attached to the base of her feathers, try to remove them. Those are clumps of eggs. I bought a bird once and did not ck her at pt of purpose. But saw all this once home--eek. Got her cleaned up so she would not infect other(you can have one bird infected and not the others too!!)
Also you'll need to do some cleaning in your coop. It's a pain but worth the effort. Take out ALL the bedding, even in nest boxes. With a shop vac, vac out all corners and cracks.if you have wood boosts, I'd paint them with veg oil so you'll take care of any mite situation there too. After the coop is clean, sprinkle/dust floor and nest boxes with DE(diatomaceous earth) which will kill any larvae. Any farm supply sells it.Dump fresh bedding on top and in boxes. I even wouldspray some Adams in the nest box bedding