You can pip the shell of late hatchers. But wait till 24 hours after everyone else has hatched to try and help weak chicks out of thier shell. Candle the egg to find air sac, and take nail or sharp pointed object and *carefully* peck tiny hole in the big end. It will not crack like a fresh egg, the shell has developed so it is thin and just soft enough to pip easily. Now you know your chick has air.
Peck out hole only big enough to see if there is movement. If you draw blood *stop*. Do not break off chunks of shell or chick will quickly dry out and die. The chick is in a membrane inside and has to draw out of it all blood and fluid before hatching. If you tear the membrane it might bleed to death, so be careful. If the membrane is dry and crusty looking, you have a "stuck chick", caused by low humidity or you opening and closing incubator. Wrap warm moist papertowels around egg, but not completely. Chick need moisture, but air too. Once chick gets to the point head and beak is outside membrane breathing air, you can take a syringe or eye dropper and very carefully drop warm water in the shell. Don't drown him. Then wait and hour or so and he should be able to come out on his own. Always let chick work, they must push against shell do develop muscles. If you just pull them out of the shell, they will never stand. Stuck on parts of shell membrane come off easy with water.
They say "Do not open hatcher until every chick has hatched", and that is true to a point. But you have to get the first, fiesty ones out of there or they will roll the other eggs around and mess them up. The hatch may last 2-3 days. Help late hatchers on day 3, or when all is quite except for one or two puny peeps. You can check back every few hours and help as needed. Be careful you don't let temp fluctuate too much opening and closing bator. I keep heater in the room and "plan" for each opening and closing if using a styrofoam model. Leaving one or two chicks peeping in the incubator helps encourage weak ones too.
Good luck,
Jeanne