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