calcium for chickens

[varmit chicken]

Hi
I have had chickens for a couple of years now, and they always have calcium available but they don't seem to eat it. And every once in a while we get a egg with no shell I have heard this is because of a lack of calcium in there diet.... Any suggestions would be greatly appreciated.
Thank you much

 

[Bear Foot Farm]

It could just be a problem with an individual chicken, since most of the eggs are normal