My chickens seem to be frequently laying eggs with very soft shells. I am feeding them the same layer crumbles they have always eaten and I don't think anything has changed in what they are eating in the run. It is not just one chicken doing it repeatedly, either, since every now and then my one Ameracauna lays a blue soft shell egg--the other chickens lay brown eggs. Has anyone encountered this before? Any suggestions on how to correct the situation? Many thanks!