Aw. rats. Something must have changed where she decided she needed to pluck instead of doing whatever she's been doing while not plucking.
I'm kind of late to the discussion and don't know anything about her or your other flock, so forgive me if I'm asking things that have been answered.
Do you think she eats an optimum diet for her species? Does she have a good appetite and eat mostly healthy fresh foods?
Does she play and keep herself busy chewing things up, or is she more apt to sit and preen/pull?
If she hasn't lived with you too long she may be unsure of the routine and turn to plucking to soothe herself if she is worried. It might take more time for her to feel secure and know that her life is stable. Since she *can* grow feathers, I think you should be optimistic that eventually she will.
If I were you, I'd overload her with items to chew/destroy/climb on. The bigger the mess she can make the busier she'll be and the less time she'll have to pluck. I've given my birds half a watermellon and just let them go at it. I bring in branches with leaves on them and they snip them all off. I constantly gather pinecones and keep the playtop on the cage full of them. Paperback books are tossed on my cagetops also.
Six months ago I would have agreed that there is likely a physical condition that is causing her to pluck. Now, after living with two pluckers/barberers, I'm not convinced. Sure there may have been a physical cause orginally but after that resolved itself I think habit might have taken over.
The very, very best diet is IMO the first course of action. Followed by daily showers. Followed by constant chewing/destruction of items.
If you decide to try the vest I caution you to not leave it on for too long without giving her a break to preen naturally. Some parrots are very unhappy in vests.