I’ll start off with a silly question. Are you counting the days right? It’s a common mistake. An egg does not have a day’s worth of development 2 seconds or 2 hours after it is placed in the incubator. It takes 24 hours for it to have a day’s worth of development. A good way to check your counting is that the day of the week you set them is the day of the week chicken eggs should hatch. If you set them on a Friday, they should hatch on a Friday.
That’s not really as important as some people think. There are a lot of reasons the eggs might hatch early or late. Heredity, humidity, how and how long they are stored before you start them, and just differences in individual eggs all play a part. A really big one is average incubating temperature. If your incubator is running a bit warm, they can be early. I’ve had eggs pipping when I went into lockdown the first time I used mine because it was a bit warm. They hatched fine. If your incubator is running a bit cool, they can be quite late. I’ve had eggs under a broody hen hatch two full days early more than once. It’s really not that exact.
I’ve also had chicks hatch a full day before the others. I’ve had a chick hatch a full day after the rest had hatched. That kind of stuff is not unusual. I always figure if they hatch within 24 hours either side of the 21 days, they are right on time. A lot of times, mine are not right on time.
Hatching is not an instantaneous process. The chick has to internal pip, external pip, then zip and come out. While they are doing this they are learning to breathe air instead of living in a liquid environment, drying up blood vessels they no longer need, absorbing the yolk, doing something with that gunk they have been living in so they dry fluffy instead of all matted down, and who knows what else. Some do a lot of this before external pip. Some do a lot after external pip and before zip. Some haven’t even finished at zip. I can’t tell what stage they are at if they are still in the shell.
I tend to leave them alone when they are hatching. There is very little if anything I can do to help and often I do more harm than good.