University of Florida research came up with this list describing possible causes for poor hatches:

18-21 day failure can be, in order of likelihood, the following:
1. Improper incubator temperature, humidity, turning, ventilation.
2. Improper hatcher temperature, humidity, ventilation.
3. Contamination, especially from molds (aspergillis, etc.).
4. Fumigation too severe or too prolonged.
5. Eggs chilled in transfer, or transferred too late.
6. Broken shell -- pre-set, during incubation, or at transfer.
7. Nutritional deficiencies -- vitamin D, vitamin A, folic acid, or pantothenic acid, riboflavin, vitamin E, selenium, vitamin K, biotin, thiamin, vitamin B12, calcium,
phosphorus, manganese, or linoleic acid.
8. Embryonic malposition; embryo fails to move into proper hatching position.
9. Embryological development accident. Failure to change to lung respiration and all intra-embryonic circulation, and/or to retract the intestinal loops and yolk sac.
These and other changes are critical at this time.
10. Heredity -- lethal genes, chromosome abnormalities.
11. Twinning (2 chicks in one egg).
12. Hatcher opened too much during pipping and hatching.
13. Poor shell quality.
14. Breeder diseases.

Pipped. Full-term embryo, dead in shell.
1. Low humidity or temperature for a prolonged period.
2. Low humidity during hatching.
3. High temperature during hatching.
4. Nutritional deficiencies.
5. Breeder diseases.
6. Poor ventilation.
7. Inadequate turning during first 12 days.
8. Injury during transfer.
9. Prolonged egg storage.

Shell partially pipped, embryo alive or dead.
1.  Excessive fumigation during hatching.
2.  Eggs set small end up.


Occasionally you may see a 'blood spot' in an egg.  It doesn't mean that the egg is fertile (although they are, as I keep roosters with all my
flocks) - it is just a bit of blood from the hen when the egg popped out of the original ovarian follicle. It lets you know the egg came from a real live hen!  I've heard
that blood spots are hereditary, which is why you rarely see them in commercial chicken eggs these days (they've been bred out).  Also they are more frequent in
brown egg layers.  Both chemically and nutritionally, these eggs are fit to eat. The spot can be removed with the tip of a knife if it bothers you, and the egg used.
It will disappear when the egg is cooked... USDA regulations, however, classify eggs with blood or meat spots as inedible. Less than 1% of all eggs produced have
blood spots. Blood spots result from hemorrhage of a small blood vessel in the ovary or oviduct. If the blood spot is on the yolk, the hemorrhage was probably in the
ovary at the time of ovulation or in the infundibulum part of the oviduct before albumen was laid down. If the blood spot is in the albumen, the hemorrhage probably
occurred in the wall of the magnum part of the oviduct. Meat spots are degenerated blood spots, loose pieces of ovary or oviduct tissue, or cuticle remnants swept
up to the magnum and included in the albumen. Leghorn strains vary in the number of eggs they lay with blood spots. Eggs from brown-egg layers will usually show a
higher incidence of blood and meat spots than those from white-egg strains. Ambient temperature has also been shown to have an effect on the incidence of blood
spots. Fewer blood spots have been observed with Leghorn hens at 32°C (89.6°F) than with Leghorn hens at 21°C (69.8°F).


Here is an online reply from an internet chicken forum about winter egg production written by Dr B (Jess).

Quick answer to the getting eggs in winter- older birds will naturally stop laying eggs in the winter- it is a natural break for them. If you want eggs during the
winter from hens who are over a year old, but otherwise healthy- you need to artificially light the coop. Also molting birds won't lay- they are putting their energy
into growing feathers, and for some reason- many hens like to go into molt in the fall- so there is a double signal to them not to lay.

Commercial egg laying facilities keep the birds on artificial light cycles so they never see short winter days. Plus they don't have older birds. This is the only
reason people can buy commercially produced eggs during the winter.

Most hobby back yard folks get used to a dramatic winter decrease in egg production, knowing they will pick back up in the spring as soon as the days start to
lengthen again. Mine are starting up again. :)
A side note on this--- how Dr B gets eggs during the winter without artificial light... add about 6 pullets every year- timing it for them to come into lay in the early
fall, so they are already laying strong when the days start to shorten. My older girls do not lay during the winter. My teenagers do. This is why I have *a bunch* of
chickens. This only works if you have space to house an large aging flock. This winter I got 4-6 eggs daily- pretty much all of which were from my youngest birds.
Now we are over a dozen again, slowly climbing. :) jess