Well, I guess a few qustions are in order. What variety? (chicken, quail, turkey, duck goose?) Do you change out water daily, and keep it full? What are you feeding? (no whole grains w/o grit, no greens the first week and only w/grit) Do you know what temperature your brooder is at? Is it meticulously cleaned between broods? If there was diesease, and you didin't bleach, you'll pass it on.....
When they hatch, I personally use disposable boxes, and use old rags and folded absorbant toweling for non-slip bedding. I use a 40 watt lightbulb for a box thats 16 x 16 appx. It is just inside, and I watch that they aren't either clumped under it (too cold) or pressed against the sides of the box (too hot). Clean feeders and waterers daily. Don't let them pick their own poo, change boxes and bedding as needed. Burn the old, or dispose of, so as not to spread illness.
Be sure your feed is unmediated with all but chickens.