2 Budgies, 1 Chest Going Bald

I am new to this site, Hello Guys.

I have 2 budgies, Oscar se is 4 and oakley he is 1. This past week i have notices that Oscar has bee losing her chest feathers andis now going bald. Oakley is showing no signs of this happening yet. Oscar is a happy budgie and they have toys to pay with and are not in a draughty place or where they are being scared. Can anyone shed any light on it please. I have a picture but do not know how to attach it

Man thanks