If the shell is still lying about, they may not be eating the eggs for the calcium, but the taste, or the protein. When my hens are getting bare backed from the roosters, or are molting, they want eggs.
I feed my hens their raw eggs all the time, and I rarely see them leave the shell behind. If the shell is always behind, you might want to look into the possibility of a possum or other animal coming in and eating the eggs. They do not eat the shells.