I am not an expert either but I would assume that dog hair is like human hair in the sense that some people just can't grow long hair.
As long as she is eating a quality dog food and being brushed regularly (brush enough to get knots out) then I'm not sure there is anything else you can do to help the hair grow. Maybe someone else that knows more than me will come along and post. You might try the search function near the top of the page to see if anyone else has had similar concerns.
Oh, and Welcome to YT!
