I saw another grey like that and I asked about her... They said that she doesn't pluck, but chews her feathers off, leaving her down feathers alone.
They said that she doesn't like change and was a nervous bird...
And that they worked with her and she got all her feathers back, but then someone had a baby and there went all her feathers again!
you might see a bird trainer? I have one and he is absolutely wonderful!