They could be late but if they did fail to hatch then it is probably a moisture issue. They could drown if your humidity is really high. More likely the membrane dried out and then it gets too tough to break. If you have a small bator you should not open the incubator or remove eggs to candle from day 18 until they are done hatching. It can kill your entire hatch by drying out the membranes. In large incubators people can manage to candle the eggs without removing them or if you know it's really humid in your area that day you can get away with it. As a general rule it's best not to. I don't even open my bators to add water after day 18. If they need more water I add it through the vent holes with airline tubing (can be purchased with the fish tank supplies) and a syringe. A long straw can be used too.
For now I would make sure your humidity is high but not fogging over your incubator windows and leave them alone for another day or 2.