My 7 year daughters hair will not grow.

I have a 7 year old daughter. She has curly hair but it can't seem to grow past shoulder length.
She hasn't had a hair cut in years and I swear it hasn't grown much in the past 2 years.
It seems to fall out more easily when I'm brushing it too. The ends look so tatty but if I cut the ends she would have a boy like hair cut. Which she would never allow to happen. (When she 3-4 years I tried the trim ever few months tip but it didn't help. It just kept getting shorter each cut till she looks like a boy so I stopped.
She has little sisters and their hair grows normally. Her 5 year old sisters hair would be past her bum if I had never cut it. I have so much hair. Hairdresses make jokes about how much hair I have. They say I have enough to cover 3 peoples head. So she hasn't inherited it from me.
She used to have major eating issues which can be a cause but that's improved so much in the past 2 years but it still hasn't helped her hair growth.
I can see it really upsets her. She is a very girly girl. And would love longer hair. Enough to braid and be able to do come hairstyles with
What can we do?
Is there actual medical conditions that can cause this? Should she be checked by the dr?

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