Yes, some eggs need a bit longer, and hatch sooner, we've had quite a few day 20 eggs before, and we just had a friend have one hatch on day 19! It wasn't a bantam breed or silkie either. We've hatched quite a few batches of chicks before, and my husband calls them peeps as well, instead of pips. Lol
However, have you read many guides on hatching chicken eggs? What type of incubator are you using? They usually come with a little booklet. I am trying to not be mean, but I am a but confused and concerned. A general rule when it comes to hatching eggs, is that there is a three day "lock down period" prior to hatch day. Day 17 or 18, as a standard for 21 day average chicken eggs, is recommended as the last day to do any candling and to stop turning them. This replicates the mama hen sitting tight in anticipation of her babies hatching. At this point the chicks will be getting into hatching position within their eggs and absorbing the rest of their yolk sac. This not only gives them the immense amount of energy require to hatch, but sustains them for the first 48 hours or so. That way they don't need to eat and drink for that time immediately after hatch. They can just focus on staying warm and resting up. Also, the humidity needs to be kept higher for those last days of incubation. Opening the incubator can mess with that because of the sudden air pressure change and moisture loss. So you don't want to do that unless there is some sort of emergency. Especially if an egg is already pipped! If you open the incubator then, you can essentially shrink-wrap the chick within it's membrane, which most often will result in death for them. After hatching you usually leave them in the incubator for at least a 12- 24 hour period as well, so that they can dry out fully. As newly hatched chicks can chill very very easily. You are obviously still having healthy chicks hatch, they are generally hardy little things. I just wanted to put this out there to make sure that you aren't inadvertently making hatching more difficult for your little chooks! If you need more information, you are already in the correct place. Backyardchickens, has a bottomless well or knowlege on it. Wishing you the best!