Unfortunately a lot of folks selling hatching eggs assume that if they're running a cockerel with a small number of hens then the eggs will be fertile. I wonder how many set a test hatch regularly through the season, as we do? There are so many factors that can affect fertility, including age of the birds, diet, stress factors, disease and so on, that it's entirely unrealistic to expect every cockerel and every hen to be fertile for the whole of their breeding lives.