Holy moly i'll answer the best I can! OK let me start off by saying that I started this as a 1st grade class project and we had problems and only hatched 5. So I decided to try at home and now for the answers Still Air Incubator, 99 temp, humidity???, americana and red sex link cross, Saturday was 21 days and no I didn't candle the eggs.
The temp in a still air bator needs to be around 101.5-102 with humidity around 50% then I raise to 65% during hatching. If the humidity was not high enough, the shells could have dried out making it hard for the chick to hatch. They get tired and die.
Don't give up, always try again. We only learn by doing. Good luck!