My son is hatching chicks for the first time in the incubator. He only put two eggs in for the first clutch they were due to hatch about 5 days ago so when they hadn't hatched today, my brother who has hatched a lot of eggs came over and told me that they definetly should have hatched by now. He carefully cracked them and the chicks were almost fully developed but dead they had not turned toward the air cell yet but were almost fully developed. My brother said he thought they had maybe died a day or two before the hatch date. What could have caused this?