What should you do when eggs break too easily? First, check your facilities and equipment to ensure it is not mechanical damage. I have seen considerable effort being wasted in examining ingredients and formulas when the real problem was a faulty piece of equipment.
Second, check for overall flock health. Your veterinarian will consult you regarding what diseases can upset the calcium cycle. Impaired gut health can impede calcium absorption, a common problem.
Then, and only then, start thinking about nutrition. It is imperative to find a nutritionist who understands the delicate daily calcium cycle and the factors that affect it. To this end, I can recommend an article that I wrote about this subject, Understanding daily calcium cycle in layer hens.
Also, other nutritional factors can cause eggs to be laid with a thin eggshell. Here is another interesting review of these issues that might be of some help understanding how nutrition interacts with egg quality, 12 ways calcium prevents cracked eggs. It also pays to remember that limestone is not always just calcium carbonate. Limestone, an inexpensive source of calcium (in the form of calcium carbonate), can vary widely in its calcium concentration and, of course, in impurities.
Finally, we must keep in mind that egg size and eggshell strength are bound in an inverse relationship: the larger the egg, the thinner the eggshell. This is because a hen spends only two grams of calcium per egg, no more, no less, no matter what egg size is laid each day. These two grams will have to be spread over a larger surface as egg size increases with hen age. Some markets prefer extra large eggs, and this is where I have received the most questions regarding eggshell strength.
Egg quality and nutrition remains one of my favorite topics, and I have the good fortune of working with several clients worldwide. At the end of the day, most problems have to do with reduced calcium absorption or quality of the calcium sources.