It happens a lot. There are many things that affect when an egg will actualy hatch, like heredity, humidity, size of the egg, how and how long they are stored before incubation stars. But the big one is average incubating temperature. If your average incubating temperature is a bit warm, they can hatch maybe 2 to 3 days early. If it is a little cool, they can be that much late.
What I would suggest is that you be patient for a few more days. I don't know for sure that is what is happening with your eggs, but I think patience is called for.
An easy way with chicken eggs to determine when they should hatch is that 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 Tuesday, they should hatch on a Tuesday. But that is theory, not always practice. They can still be a few days off.