I would get some Corid and treat them for possible coccidosis. It is hot where they are kept? How do their droppings look? Until you get the Corid, feed the weak chick with a dropper, and give it water with electrolytes, or gatorade or pedialyte. If it will drink on it's own that will help. Symptoms of cocci are runny droppings with blood or mucus, lethargy, hunched posture, and not eating. Dosage is 2 tsp of Corid liquid, or 1 1/2 tsp of the powder per gallon of water for 5 days. Then later, give some vitamins and plain yogurt.