Do some of them just never leg eggs? Do they need to see a rooster to get them started?
I have a couple that have only ever laid a few eggs in their life time. They are 5 years old now and yes it is possible to have a sterile hen. Not common but possible.
I have a splash hamburg that is in my avatar that had a rough start in life and only lays about 30 eggs a year. The hamburg breeds are said to be super troopers on laying but she is not. The other hamburgs I have are awesome layers. Just not this one.