Why does my toe nails keep separating in layers?

Both of my big toe nails separate in layers. I have this since I was a kid. The little toes don't have this problem, only the 2 big toes. It doesn't hurt and it doesn't fall off until its fully grown and I cut it. Please advice.

Btw I've been to a doctor and took antifungals 3 years ago and it didn't help at all.