I had 16 delivered and 4 of those were not fertile. 3 of my belgian bantams, which are new layers. What makes them not fertile. I have lots to learn... I never knew I would get into chickens but I am already hooked in my second year.
Eggs that don't develop does not mean they are not fertile. Anything can cause them to not start. Some are clear from lack of fertility too.... but not ALL.