we have had some gliders we rescued/rehomed and they had nails that sound like
you describe, I think is was because the nail cuttings are to far apart.
the ones we have seemed to go back to normal after some good cuttings.
We cut every 3 weeks about and that seems like almost to long sometimes, 2 weeks
seems like the better interval.
I would describe the back nail is a half moon, and the inside of the moon is
filled in with nail material.