Allow the birds to form their own pair bonds if you have enough birds of that species. You know that you have pair bonds when the birds sit close, preen each other, and keep others away. Most of the reasons just discussed are applicable. If you hold an egg up to a bright light, a red streak will indicate if they are fertile. Of course, these will not be fertile.
Poor nutrition is an important cause of breeding problems.
Sometimes age, excessive breeding, reproductive tract defects, or diseases will cause infertile eggs to be laid. Sometimes bacterial or viral infections affect the fertile egg. Be sure that there is adequate humidity. This can just be a bath provided for the egg sitters. Sometimes the egg-sitting parents may be frightened and may sit too hard.