Are you counting the day you set them as day 1 or day 0? Most people count that as day1 but infact it's day 0. Check your humidity, make sure your in the 60 to 70% range. Don't open the bator if you have eggs that have pipped. If the humidity drops to o low before they hatch, add more to bring it up but only after checking for pipped eggs. Use warm not hot water, it will bring the humidity up faster.
Some will hatch early, some late, the majority at about the same time.
Do not unplug the bator until day 23 or 24 as they may still hatch....if by day 24 and none have hatch, then it's safe to say that none will. HTH