R.
This may sound crazy but it could possibly be a sensory issue. My son was 3 when we discovered he had it and it was not that obvious. We took him to a highly recommended Occupational Therapist and sure enough he did. We took him once a week for about 6 -9 months. He is a changed boy now. Instead of trying all the nail remedies that have been used and not successful fro years, just give this guy a call and talk to him. He will be honest with you and tell you if it's no big deal or not. I personally would rather treat the emotional reason as to why she is doing it than fight her for the rest of her life.
Eric Graffeo
Its For Kids
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4902 Creekside Drv Ste. A
Clearwater FL 33760
USA
I hope you find your answer!