Around half of the chicks actually hatch.... but I mean that the chicks die hours before they are ready to hatch. They have all their stuff absorbed into their body, fully developed, and just won't bother to hatch.
Sometimes the shell is too hard for them to break out.. If you know it is going to happen keep watch and help them to hatch.
I helped quite a few and they grew up to be healthy. keep your hands very clean. Sometimes all they need is for you to start the hole.
If you know one is having problems take the egg out and lightly tap and peep at it then put it to your ear. Where you hear the peep the loudest carefully start a hole and put it back into the incubator. If you peep at the egg every so often it will help to stimulate the chick
to hatch. When I have had give more help I take 2 clean pillowcases and tape them along the sides of my hova bator. This allows me to prop open my hova bator and get to work all the while not loosing alot of heat or moisture.