You get the picture. It is not always easy to determine why some eggs just don't hatch. In my experience, usually one or two eggs simply don't hatch. It's usually the last eggs laid. Some of my best and sweetest babies have been the runt of the clutch.
Nature knows best.
Not all eggs are meant to hatch. Obviously if none of your eggs are ever hatching, you should be concerned. Believe me, in nature all the eggs do not hatch every time. Splay legs can be very traumatic for the novice breeder. Even the best nest-builders should be given a basic substrate for the nest box.