I had the same issue this week, but I assumed it was humidity. I had a couple come out very wet and their umbilical cords were red and big! They are all fine, but I had alot that fully developed and never came out, and one that piped but never zipped. I think it drowned in the egg.
Other than humidity, I wouldn't know what to tell you other than it is just one of those things. Maybe (and this is a brainstorm here) the shells of the eggs were too thick. I have heard that too much calcium in the hens diet can make the shells very hard for the little boogers to break. Same as too little calcium and they break when you look at them.
Arrowhead07 - I am so sorry about your little ones that didn't make it, it is always so sad.
I assume that you opened the shell to check on the ones that didn't make it. I do that as well, to figure out what might have happened.
Were they still tightly wrapped, head down, or had they twisted their heads up, to get into position, yoke absorbed? Sometimes it helps to determine the cause.