I have heard that some hatchlings will pip, then go back into the egg. If they are unable to find the hole that they pipped through...they will suffocate/drown in the egg. This is just my guess as to what may have happened. I'm very sorry to hear of your loss.
That is one benefit to cutting eggs. you can make a slightly larger whole that is easier for them to find, should they venture back into the egg.
Good luck with the rest of the babies.