In my experience, it would be humidity or ventilation issues. Do you have a picture of the membranes?
I don't. My vent hole was wide open and the humidity was approx. 65%. I did have the humidity on 55% for four days during the middle of incubation while I hatched a turkey egg in there, but I can't see that being such a problem as I had 11 hatch out just fine. I'm leaning toward maybe it's an issue with the Delawares, though I got the breeding stock from Will Morrow. I should watch and see if it's the eggs from one particular hen because 6 of the 11 were full Delaware. They are vigorous and happy.