Why does my daughter have bad posture?

Although she's been taking ballet lessons for 10 years, her posture looks bad especially when she sits.
My husband and I do care about posture, and have tried to correct her posture, but it doesn't get better.
Possible reasons are;
- her legs are pronated --> because of no or less arch, wearing orthotics
- playing iPhone too much --> used to be given 'time rule' but not anymore --> rounded shoulders
- she's stubborn, easy to forget things --> her character
What else?
Does she need advice from a specialist?Why does my daughter have bad posture

If she can't balance a book on her head with practice, you might want to see a doctor.