Had a hatch over the last couple days that we expected 31 out of 39 (by candling) and only ended up with 15. We opened the remaining eggs as candling showed they were gone and 16 were nearly or fully developed but never broke the membrane into the air cell. Our humidity was 25-35% for the first 19 days and 60-75% for the remainder. The temperature was steady at 99.5 with maybe half degree fluctuation either direction (2 thermometers in the bator- one digital, one scientific- both in a water wiggler). The 15 that hatched are all healthy and running around. What could have caused so many fully formed chicks to never enter the air sac?? Please help!!