I'm not sure if i can help, but I'll try. Some questions: Are the birds closely related?
I had the same problem this Spring with a pair of Rock bantams. The eggs just wouldn't hatch. Other eggs in the same batch did so I assumed something was wrong genetically.
Is your bator still air or forced? Are u using a turner? With a still air, the temps vary. When u take eggs off turner and place them lower, the temp change can get them. (I've heard.)
Humidity could be too high. How big are the air cells?
My incubator is the new digital forced air Genesis. I have hatched out 4 hatches of eggs with a 95% hatch rate before attempting to hatch modern bantam eggs. As for the birds related,I don't know, I got them from Ideal Hatchery. I turn my eggs myself. The air cells was the same size of the cornish eggs that hatched. The only thing I can think of is they require a different humidity to hatch out. I have 6 more to set to hatch out on 11/09. I hope some of them hatch.
Mark