We've got a friend that has been incubating eggs for us. The first batch went great when out of 8 eggs we got 7 birds. This last batch though we handed over 15 eggs and after 23 days only 4 hatched. When we broke open the other eggs each one had a fully developed chick inside, one or two had their intestines and things on the outside of them but other than those the rest were fine. What causes a fully developed chick not to come out of the shell? With the next batch should we crack the shell for them once they start moving around? Drill a little hole to help them out maybe?