California quail keep dying, i'm i doing something wrong?

i hatched 7 californa quail earlier this year and slowly one by one they have all died bar 1. They aren't undenurished, have had a good weight, they don't have fleas, mites, worms or anything else. They are fed on a good natural diet. I hatched them under a small bantam, my little barbu d'anver hen, so they haven't squashed. It's like for no reason at all they've just keeled over dead. Does anybody know if i'm doing something wrong, or if it's to do with the type of quail or if it's to do with their parentage?