I have 7 hens of various breeds, all laying nicely. Most of the eggs are very smooth similar to store-bought eggs, but a few have very rough shells that feel like sandpaper. Does this indicate a dietary deficiency or is it normal? They are on layer feed, plus finely crushed eggshells and they get many supplemental veggies, etc. Also, the membrane seems very tough on most of the eggs. I have to WORK to crack them!