My 14 yr old daughter's big toenails each have a crack across them at the base, just past the cuticle. It looks like the top layer of nail is detaching from the bottom. this happened several months ago and I thought it was trauma from loose shoes - changed the shoes - now that's grown out (the top nail coming off and a healthy nail underneath) it is happening again. We have a healthy, balanced diet with fish, meat, fruit, veg, cod liver oil, the works. What could this be? Thanks.