Eggs don't always hatch at exactly 21 days. They can be a couple of days early or a couple opf days late, maybe even more. Many things affect when they hatch, heredity, humidity, size of the egg for the breed (small eggs tend to hatch a bit early, large eggs late), how and how long they were stored before you started incubating. Some more I can't think of right now. But the big thing is average incubating temperature. If your average incubating temperature is a bit high they can be a couple of days early. If it is just a bit low, they can be late. I had eggs already pipping when I went into lockdown my first couple of incubations before i got my temperature adjusted. It's not that big a deal.
Another thing that can mess up when they hatch is that a lot of people count the days wrong. Some people start counting the day they put them in the incubator. That's not right. It takes 24 hours for an egg to have a day's worth of development. An easy way to figure when they shoudl hatch is the day of the week you put them in is the day of the week they should hatch. If you put them in on a Friday, they should hatch on a Friday. But that is just that they should hatch. They can still actually hatch a day or two early or late.
Hatching is not an instantaneous process either. They internal pip, external pip, and finally zip and come out. Part of hatching is that the chick has to learn to breathe in air instead breathing in a liquid, they have to absorb blood vessels they no longer need, they need to absorb the yolk to live off of for a few days, they need to somehow dry that messy liquid they are living in so they dry fluffy instead of with the down all matted down, and who knows what else is going on. Some chicks do a lot of this between internal pip and external pip. I like these because it usually doesn't take long after they external pip until they zip. Some do a lot between external pip and zip. These can drive you batty because they take so long to zip after they pip. Occasionally you get one that hasn't quite finished everything before it zips. Most of these make it fine.
Sometimes you can hear the chicks peeping after they internal pip and before they external pip. Sometimes, but not always. Often they will peep if you tap on the incubator to get their attention. Often, but not always. It is an exciting time but it can also be nervewracking.
Good luck!!!