Check to make sure your brooder is not too warm. Often a hot brooder will bring on pasty butt, and stress the chicks out leading to early deaths. Look for foamy poop, brighter yellow and running when first out, that helps too, although some of that can be normal if you see it all the time the chicks are stressed.
The best gauge of the chicks temperature needs is them. I don't totally trust thermometers and batches of chicks can vary in needs, as can the brooder vary from time of day or night. I always make sure there is a way the chicks can get away from the heat, and also be right under it. So if you start by checking by thermometer I'd say have the area right under the lamp a bit warmer than the temp for their age, and areas away in the corners too cool. If you then see all the chicks hanging out in the corners you know they are not liking the hotter area and need to reduce that. Keep going till they spread out even, going from warmer to cooler as they wish and seem active and happy.
The other option is an 'electric hen' set up where they can come and go from the warm area and be out in the cool as they desire.