The humidity could have been too low, but sometimes its hard to be sure because incubating eggs can be complicated. Sometimes the eggs are too small or of the wrong shape, or the chicks were just too weak to hatch. I've unfortunately had a lot of chicks, in incubators and under their mothers, that couldn't make it out on their own.
As for helping chicks out of shells, we learned the hard way why this generally doesn't work. Chicks we've helped out of the shell have been eaten by their mother, died from being too weak or under-aged, or developed spraddle leg.
Still, when you hear them peeping but see no progress you can't help but give them a little assistance, and sometimes it does work! We use a needle to very, very carefully break away the shell around the beak area (where the chick would naturally break out). If the chicks haven't broken through the egg then carefully pick away the shell around where you guess their beak is (listen for the sound of tapping or cheeping), but try not to tear the membrane, as this can cause bleeding.
Wait about 10 minutes or so and test the membrane. If they bleed when this is poked (careful not to poke the chick) then you should wait a little while longer, to let them absorb the blood. Then you can help break a ring around the shell, as the chick would do if it were naturally hatching.
Let them do some of the work if possible, as it strengths them. Don't rush the process, but don't wait too long, as we have waited to long on multiple occasions and lost chicks because of it. Eventually they must not be able to get enough air or something and die, so make sure they can breath.
Chicks that can't hatch on their own shouldn't be bred, as they can pass this on to their own chicks, but they can make wonderful layers, pets, and even show birds.
Best of luck with your chicks!
