Just posted this in another thread...but here goes:
 
 
Lately, seems like I have heard a lot about shell-less and thin shelled. 
 
 I had some for awhile.    I added vitamin D-3, and tried to boost calcium too.  My shells from the particular hen have been good lately, although today's had a puncture, not sure if it was a foot going through or if she is trying to eat eggs.
 
apple cider vinegar helps the chicken's digestive system have the pH to absorb nutrients, like calcium.  1-tablespoon to 1-gallon of water.
 
Lots of crushed oyster shell free choice, their own egg shells crushed and fed back to them if you choose to.
 
I take a D-3 tablet and crush it on the counter, mix it with some feed that I put some water on to make sure that the D-3 powder will stick.  Guess I should say vitamin D-3 -- which I got at 
Walmart--it helps them with shell formation. 
 
Use your layer ration that contains the most calcium and hold off on treats that don't contain calcium.  It takes a little while but that may help the shell problem.