There are many reasons why fertile eggs wonít hatch. When an embryo dies early in itís growth, it is called addled. Addled eggs can be caused from insufficiency of vitamins, proteins and minerals. Another cause of addled eggs is when inbreeding has occurred. Inbreeding will inadvertently introduce recessive genes that are responsible for debilitating, deforming or even killing the chick at an early age. For more information on on inbreeding, read this article.
If an embryo dies later on, it is considered dead in shell. Dead in shell is usually caused by the nest box being too dry. If the box is too dry, incubating eggs lose too much moisture. The chick inside the egg becomes weakened and cannot break out of the egg. However, usually most parent birds will bathe before sitting on their eggs. This will give the eggs the moisture they need.
Eggs that have been infected by bacteria are usually the consequence of the egg shell being cracked or the shell being too porous. Eggs may get cracked if there is insufficient bedding in the nest box, or when birds are easily frightened and disturbed while they are inside the box. A possible cause of eggs being porous is the spraying of chemicals. Spraying of insecticides or any other harmful household product in the same air space as the birds or their food and water.
Another cause of embryo death due to bacterial infection is the egg yolk. If the hen is passing along bacteria through her uterus to the egg yolk, it can cause embryo or early chick death.
Some of the information on this subject was taken from "Encyclopedia of Cockatiels" written by George A. Smith © 1978 by T.F.H. Publications, Inc. LTD