I got my chickens at a day old. I am now collecting eggs and selling them. In the cold weather of a Canadian winter no less! what made one egg's shell shatter? It was soft and I put my thumb through it.
Definitely offer up more calcium as stated in the above post. Also this being winter, birds are not in tip top shape and can lay mishappen eggs, thin shells, etc... Make sure they are on a good diet, get lots of fresh air even when it's cold outside and provide oyster shell on the side.
Soft and thin shells isn't always about a need for calcium. Ca to phosphorus ratio has an impact. Vitamin D3 must be present in the right amount.
Body chemistry, including acidity affect the ability to process calcium.