I have a few pullets that are starting to lay eggs. Many have a soft shell or no shell at all. For three weeks now I have been mixing oyster shell with the food along with oyster shell out of the coop to pick at. My pullets seem to want nothing to do with the oyster shell at all. Is there an alternative to oyster shell to make sure they are getting enough calcium? Oh and they have been eating layer feed for over a month now and I thought that would provide enough calcium. Thank you for any advice!