Start with the heat temp. For one week old hatchlings, temp should be around 97 the first week, lowered by 5 degrees each week. Make sure there are no drafts in the brooder. Keep the heat source in the center of the brooder so they can use the heat or get out from under it. Make sure there is enough air in the brooder and you are not using a air tight lid.
Make sure to use a shallow water dish so they do not drown. If they are hatchlings, make sure they know where the food and water is by dipping each beak in the water and sprinkling food around the floor. Make sure food is not moldy or stale.
If they are older than a few days, make sure the water and feed areas are elevated so it is dry and poop free in their water and food bowls.
Check the poop color. If there is any blood in the poop, they may have cocci. It helps to raise quail on wire if you have an outbreak of cocci. (poop falls thru the wire and they have less contact with it). Definitely elevate the water and feed areas with cocci. If you suspect an outbreak, use Corid or switch to a medicated feed.
Do not over crowd them. Some may be getting trampled, can't get to the water or feed. Use 2 brooders if the one you have is too small. Use several feeding and water stations so everybody eats and drinks.
If all this is being done, it could be something that "grew" in the incubator, something passed down into the egg or some sort of virus or bacteria that they contracted.