Why does the canary lose feathers on its head?

Hello everyone! My cousin's canary has almost no feathers left on its head, and it's been that way for over a year. The bird seems relatively lively, though, just looks very plucked. The owners have tried many things, but nothing has helped. It's unlikely that the bird is suffering from a mite infestation, as the condition hasn't improved despite treatment (drops on the neck). I once read something about a genetic defect or a hormonal imbalance that could be responsible for the feather loss. Unfortunately, I don't know anything about canaries and wanted to know if anyone knows what else can be done or what the cause could be. The feather loss is exclusively on the head, and the bird also sneezes strangely sometimes. The female bird is about four years old and lives with a male (roughly the same age). They regularly get free flights, greens, minerals, etc. The male is completely healthy, and the two get along quite well. Thanks in advance! 🙂