Are the shells hard? If shells are thin or weak, a hen will easily peck the shell and it will break. Once it breaks or cracks, you can't stop them from smelling (and eating) all that gooey warm stuff inside. A farmer once told me that a hen will lay an egg, then she'll tap it with her beak to test the shell's strength. If the shell breaks, it was too thin or weak and would not have sustained a developing embryo. He said she's just improving her odds for healthy chicks.
For strong shells, make sure your chickens have access to oyster shell, in addition to their regular feed. And add ACV to their water (plastic waterer, not metal) to help with their calcium uptake. 1 Tablespoon per gallon, on the ACV.