I sympathise Heather. I have the same problem. Mine free range and are on mixed corn.
I have, however, temporarily overcome the problem. I have a broody hen sitting on her own in the stable. The others go in there to lay their eggs, usually sitting on top of the broody. She then scoops the egg under her and keeps it safe. Well at least until I take it from her. Poor broody, we don't have a cockerel so they will never hatch anyway.