Eggs won't hatch

I am new to this so maybe I'm doing something wrong. I have 12 buff orpington hens and one buff rooster. They are all now 3 years old. I have tried 5 different times with 5 different hens to hatch a brood of eggs. But I usually get nothing. One time last year I got some incompletely developed chicks because my husband disturbed the hen. But all of the 4 other times I patiently wait the 21 days. Then I usually wait 4 or 5 extra days just for good measure. Then I take the eggs and candle them, but there is never anything other than egg inside them. My rooster acts like a rooster, he crows and struts and defends his hens with a passion. The coop is well ventilated. The nests are kept clean and filled with a deep bedding of straw. I feed Purina feed. The water is kept clean and full. They are allowed to free range if they want, although the brooding hens stay on their boxes. I let them start brooding after the danger of frost is past, but before it gets real hot. So what am I doing wrong? Why don't I have baby chickens running around?