incorrect humidity is one of the most common reasons
http://www.brinsea.com/Articles/Advice/Humidity.aspx
The effect of humidity upon the incubating egg
Egg shells are porous - they allow water to pass through, and so all eggs, whether being incubated or not, dry out slowly. The amount of water that an egg loses during incubation is important and this is determined by the humidity levels within an incubator; if the humidity level is higher then the egg will ‘dry out’ more slowly than if the humidity is lower.
All eggs have an air space at the round end and as water is lost through the shell it is replaced by air drawn through the shell into the air space which gradually increases in size – the greater the water loss through the shell, the larger the airspace. This air space plays a crucial part in incubation. Within it is the first air that the fully developed chick breathes and the space allows the developed chick some movement inside the shell to allow it to maneuver into hatching position.
If the incubation humidity has been too high the egg will have lost too little moisture and the chick
will be rather large. In this case the air space will be too small, the chick’s respiration will be affected and the young bird will have difficulty breaking out of the shell because of the lack of space. Commonly with excessive incubation humidity the chicks will die having broken through the shell in one place (‘pipped’) either through weakness because of the lack of air to breathe in the shell or because of lack of space to turn and cut around the shell with their bill. Often, because of pressure within the egg, the bill protrudes too far out of the initial hole preventing the normal anti-clockwise progress of the bill chipping the shell from inside. The bill becomes gummed up with drying mucus.
Low incubation humidity levels lead to small chicks with large air spaces by the time the hatch is due. These chicks will tend to be weak and may also die just before, during or just after hatching.
It should be noted that in general that a slightly lower humidity level than optimum is likely to be less disastrous than a slightly higher than ideal level.
