Hello Beverly, I'm sorry to hear about Prissy. Given that none of the others are showing similar symptoms there are a few possibilities for her condition. One of the first issues I'd address is food. Often feather loss or self plucking like this is the result of what's called hypovitaminosis A, or simply put, vitamin A deficiency Even the foods with additives won't address it; in fact, save your money, none of those things actually work. Why? Because 'keets don't ingest the whole seed. All the additives are sprayed ON to the seeds and get left behind with the hulls as the birds eat the inner piece.
Now, what many owners don't know is that in nature these birds not only don't eat just seeds, but a seed only diet is like a human eating nothing but, let's say, french fries. It's a high fat diet with minimal nutritional options, but just like we like our high fat, good tasting foods despite them being bad for us when it's all we eat, birds aren't different.
Birds also have the option in nature to fly miles throughout the day in search of food so the high fat seeds are metabolized quickly. In captivity there are no miles of foraging to be had.
Ideally, domesticated birds are better off on a pelleted diet supplemented by fresh foods like kale, carrots/carrot tops, sweet potatoes are really well received (I use all natural baby food when converting rescued birds to a pellet and fresh food diet)
Dark green, dark orange foods are generally good options, but yes, they require a lot more attention from the human in purchase and prep and yes, there will be plenty of waste, especially at first.
ANY pellet diet is better than no pellet diet - but the better ones are the no added colors or flavors, again, like we humans should have.
So, to address your issue immediately, if possible separate at this one from the others so you can focus on her supplementation. If she's bonded with her male he should come too to prevent stress.
There are recipe ideas for conversions that actually work here:
Since you've caught this early it shouldn't be a habit yet and you might see results in a matter of a week or two. Keep trying.
Other issues might be internal parasites or infections such as bacterial or fungal which really can't be treated without Rx's. No pet store med will actually work on birds and can often kill them. Unfortunately there is no 'truth in advertising' with regard to our animals like with humans.
Having her isolated from the others will also give you opportunity to note her droppings. If they are watery, yellowish or otherwise discolored, or have an odor it's time to see your avian specific vet. These are symptoms one cannot ignore.
Check back with me as you go through your options ok? I'll work with you all the way