My theory is sometimes they just don't develop properly and it can take a few days for that to become apparent once they hatch. I don't think you did anything wrong. It is tough though when you only have a small number of chicks hatch out. :hugs
Every time you put your hand in the brooder, tap the food to encourage him to eat. Have you ground the food up finely? They need it fine/powdery for quite some time as they are so tiny. He should be either eating or sleeping. That's pretty much what quail chicks do and without siblings or...