I am so sorry about your babies.
Don't have any ideas of what could be happening.
On my first broody, she had 3 eggs and only hatched 1 chick. And he/she is a cutie. Right now I have one Black Australorp with 4 babies and still sitting on two eggs. The first one hatched on Saturday. And then I have the 3 bantams that were all sharing a nest with 5 standard eggs and they have hatched out 3 of the eggs. Then this morning I found another egg had hatched but the chick was dead, so I gave the last egg to my BA and it hatched out this afternoon. Now I am not sure what is happening with her other 2 eggs, but decided to leave them a few more days as they were late comers to the nest. I sure hope I don't find and green gooey stuff in them. That must smell terrible