Hi.
What are you feeding them and do you have plenty of nesting material in the nest boxes. Hens lay standing up, so if the shells are fragile perhaps due to incorrect diet and there is little or no nesting material, then as the egg comes out and hits the ground it will most likely dent the end. If they are not on layers feed, then ensure they have access to plenty of calcium. Dried crushed egg shells and crushed oyster shells will provide extra calcium for them. They should have free access to it so that they can get as much or as little as they need.
Hope that helps
Regards
Barbara