Why is my zebra finch bald?

I agree with the previous post regarding feather plucking but with finch I usually have found it more than not that a cage mate is doing it to them. Watch and see if someone is picking on her. Once the feathers have been plucked for a long period of time the follicle is damaged and the feather will not grow back, that is what happens when you see these birds that are severely plucked. I have two almost bald babies in my home that plucked to that point while in abusive situations. Good luck but don't worry too much unless you see that harm is coming to her from her cage mates.