How old is this bird? It is likely that she is molting. However, it could be that the other birds are pulling out her tail feathers. I'd observe her with your flock, and watch for any bullying. If you're worried about mites/lice, perform a simple parasite inspection. Here is how: Grab your bird firmly. Then, part the feathers near the vent, neck, base of tail, and under the wings so that you can see the skin. If your bird has mites/lice, you will see moving black specks (mites) or larger yellow blobs (lice). Sometimes, mites will also make the feathers extremely dirty and covered with a black substance.
If your bird has mites, dust her with poultry dust, 5-10% Sevin dust, or with diatamaceous earth (though DE has never worked for me, others have remarkable success stories). Repeat the dusting 10 days after the first dusting to kill any mite/lice eggs that hatched later, and therefore were not killed.