There's usually 1 or 2 every hatch that just fail to get out. Probably a problem with the chicks. If a good portion of them fail to finish hatching though it's usually a humidity issue. Chicks are generally fairly hardly toward temp swings around that time unless it gets really hot (104f+) or stays cold for a long time. High humidity will result in drowning and you'll find eggs full of moisture and watery around the chicks. Low humidity makes the membrane too tough to break or dries the chick to the inside of the shell and you'll find chicks that look shrink wrapped in the end of the shell. Opening an incubator can also temporarily drop the humidity and if a chick is pipping the membrane then they may get stuck and die. However if it's been days since they died it can be hard to tell what the humidity was at the time they died. They could die from high humidity and then you open to remove chicks, drop the humidity, and dry the eggs so it looks like the humidity was low.