Eggs not turned often enough. Temperatures too high, too low, or too variable during incubation. Too little humidity in the incubator or occasionally too much. Then give them fresh water and feed. Rearing the chicks as a project has certain limitations, but if they are to be kept for a few days, they should be given a chick-starting mash obtainable at any feed or farm supply store.
Fresh water is also important.
Since the disposal of day-old or started chicks may be difficult, have the solution to this problem worked out before you undertake this project. If you are going to rear the chicks at home, secure your parents' permission and cooperation in advance. Cleaning the incubator. When the hatch is completed, disconnect the incubator. Remove all shells and unhatched eggs and wipe the interior clean with a soapy sponge.