I'm sorry to hear you are having this trouble. There are several reasons why this happens. One is that the eggs were never fertile to begin with. Just because you have a male and a female doesn't necessarily mean they mated. Females lay eggs whether they have mated or not. They even lay eggs when no male is present. It's also possible for the male bird to be sterile, so that even if the cockatiels mate, the eggs won't be fertile.
Another common reason for failure to hatch is improper incubation of the eggs. If the eggs are at the wrong temperature for awhile, the developing embryos die. This can happen if the birds have too much or too little nesting material, making it difficult for them to keep the eggs at the proper temperature. Inexperienced parents may not set on the eggs consistently. If the setting bird leaves the nest while you take care of the daily cleaning and feeding chores, and then doesn't go back to the nest right away, that can be enough to chill the eggs. Inadequate humidity can also cause the embryos to die in the eggs. Bacterial infections passed from the mother to the eggs can kill the embryos as well. The following site has much information on why fertile eggs might not hatch:http://www.cockatiel.org/articles/nohatch.html
This site has excellent information on breeding.http://www.cockatielcottage.net/breeding.html
Inadequate nutrition can also cause hatching problems. Diet is the single most important factor in keeping our birds healthy and able to reproduce. Many people still feed a diet consisting mostly of seeds, but recent research has shown that an all-seed diet is not good for cockatiels. Forti-diet is a seed-based diet. Pellets, supplemented with various fresh and healthy “people foods,” along with only a few seeds make up a healthier diet. Birds that are fed mostly seeds tend to develop fatty liver disease, tumors, and other health conditions. however, if your birds are used to eating mostly seeds, you’ll have to convert them to pellets gradually. The following two sites are where you can get reliable information on feeding and care. The second one has a section on converting from seeds to pellets. http://www.4animalcare.org/birdshttp://www.cockatiels.org/ownersandenthusiasts/converting-seed-junkie-to-pellets.htm
I would suggest upgrading the diet, providing a good nestbox with material in it, and trying again. If the present eggs have been incubated for three weeks or more, they will not hatch and can be removed. This may trigger the birds to try again. However, the female should not lay more than two clutches a year, so don't encourage a third. If they incubate another clutch in all the proper conditions, but the eggs don't hatch, you may want to take your birds to a vet to be examined for reproductive problems.
Checking eggs by holding them up to a bright light is called candling. This would be a way to determine what is going on in the eggs. Experienced chicken and bird breeders can candle eggs to tell if they're fertile, but it can be difficult for a beginner to identify what they see. Candling is basically shining a bright light through the egg in a dark room to see what's inside. At the large end of the egg, you should be able to see blood vessels. An embryo will appear as a dark spot in that area. If the eggs are less than about 8 days old, you won't be able to see an embryo. You can read more about how to candle, including the type of light to use, by scrolling down about halfway (past the ads) on the following page:
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