Why are my chicks full developed and not hatching

I have 9 chicken eggs in an incubator the tempiture has been 99.5 the entire time and the humity has been good. I have only had four eggs hatch but all 9 are fertile and full grown. The shells are brown so it is hard to candle but the only thing visible is the air pocket at the top on them. What has caused these eggs to go 23 days with no signs of hatching? Did i do something wrong? Can Someone please offer some help. thanks