Does she rub her butt on trees and such?That can be an issue for growing out long tails. There are several reasons horses rub their butts- pinworms which will cause discomfort under the dock, dirty teats (very itchy for them, they try to itch them by rubbing their butt), or dry itchy skin. I would say its one of the first two because her skin doesn't like dry or dand-ruffy at all. You can test if its really the docky itching by scratching it with your fingers, if she leans into or raises her tail up her dock it's probably just itchy. I would get a fecal egg test for pinworms and give her a good wormer for pinworms anyways, make sure her teats and all area in between her hind legs and butt cheeks are clean, and if those don't work spray it with a 50/50 mix of listerine mouth wash and baby oil on her tail daily.
“Good things come to those who wait… greater things come to those who get off their ass and do anything to make it happen.” - Unknown