Greetings, I am Dr. Pat. I have worked with birds exclusively for many years.
I am sorry no other expert has taken your question. We all come online at different times, I have just logged in and saw that you have not been answered. I hope I can still be of assistance.
It is very important that the cause be determined and the malady treated ASAP. They can have permanent damage. At her age I doubt if there are behavioral issues, and this is very likely physical problem. Number one on my list is intestinal protozoan infection--often associated with the signs you describe and common in breeder situations. It is fairly easy to rule out but you need an experienced vet's input and help in diagnosis and treatment. Second would be spiral bacterial infection. Again, a simple mouth swab can identify the organisms right in the office.
Feather issues can be caused by a multitude of things, including bacterial skin infection, viruses, fungal infections, allergies, metal poisoning, hormonal flux, psychological or combination of these factors. The difficulty is diagnosing the problems and assigning an intelligent treatment plan. Your vet will want to run a number of tests so that appropriate medications can be prescribed.
The challenge is to find out exactly what is going on, since treatment will depend on careful and accurate diagnosis.
Inflammatory skin/follicle disease is common. The causes can include local infection, metabolic problems, or even intestinal parasites. An avian-experienced vet should take a look at the poor bird, and run some tests.
If this were my patient, and money no object, I would start with complete fecal analysis and direct smear, stained with Sedi-stain and unstained for multiple parasites, fungi, spirals; direct smear stained with Sedi-stain and unstained of the oral cavity and feather pulp; bacterial culture and sensitivity of the feather pulp, feces and choana. Depending on the case I might do a fungal culture. Routine blood work is necessary to rule out other issues. There are MANY DNA/RNA tests for bird diseases--and testing for Borna Virus would be a good idea. Ultrasound is often more informative than radiographs and does not require anesthesia (ask your vet about this option). Generally I start them out on medications as indicated by the tests.Generally I start them out on injectable antibiotics and an anti-inflammatory such as ibuprofen.
IMPORTANT Check the website for the feed you give, there have been many recalls; or check this link: