Have you checked if they are mites? If they are, you should treat them while the chick is still young, you can buy mite spray for the chickens from trustworthy companies such as Johnsons poultry mite and lice and Nettex and Barrier. Hope this helps.
That's the same thing that happened to one of my button quails, it was only 4 days old with no shortage of food and water and I was keeping them warm, it might have been suffocation, I've seen the healthier quails sleeping on the weaker ones, suffocating them but not to death, one of the yellow...