Did you count the days right? That is a pretty common mistake on here. It takes an egg 24 hours to have a day's worth of development so you say "one" 24 hours after you set them. An easy way to check is that the day of the week you set them is the day of the week the 21 days is up. If you started them on a Friday then the 21 days was up last Friday.
Some eggs can hatch as much as 2 or even 3 days early or late and survive, whether under a broody hen or in an incubator. There are different reasons for that: heredity, humidity, how and how long they were stored, or just differences in the eggs. That's one reason you may get a day or two in between eggs hatching. As mentioned above, average incubation temperature is important too. Very important.
My incubator hatches are typically around 20 chicks. I sometimes have the hatch over within 16 hours of the first one pipping. I like those. Sometimes the hatch stretches out over 2 days, those can be stressful. I had an incubator hatch where one chick hatched on Day 19 and I did not even see a pip on another egg for 24 hours, about when I went to bed. The next morning I woke up to another 16 chicks. That's all that hatched. Each hatch is unique, you never know for sure what will happen. I've had some where a single chick was way late.
When the chicks hatch they can make a mess in the incubator. The insides of the eggs can have some liquid and the chicks start pooping. After two or three days the incubator starts to stink. When it starts to stink I stop incubating and toss the remaining eggs. Otherwise I stop on day 24. A couple of times I've had 100% hatch which takes the guesswork out of it.
