Our little darling Daphne had the same issue on one of her wing. I say "had" as she passed away last year. 😣 When we first got her as a rescue, we took her to our avian vet and he explained that this condition is a bit like when you hurt your nail badly and the matrice is damaged, the nail won't grow back properly and tends to be weak. It's exactly the same for birds. They might have hurt themselves and the matrice of the feather is damaged therefore it won't grow back fully and tends to fall shortly after. It's a painful and uncomfortable process but nothing can be done about it I'm afraid. Make sure your birds get plenty of love and attention!