We have issues with the membranes on our eggs being so tough that the chicks trying to hatch are having problems breaking through the membrane. They get through the shell but not the membrane. We had to assist our chicks in hatching today and two had already suffocated and it was really sad. Does anyone know what causes such tough membranes? We feed our hens a feed mix for laying hens that we buy at Tractor Supply. The shells and membranes are really, really hard and tough. Should we be feeding them something else?