I've found that as some layers approach molt, the quality of their eggs falls off. I think their bodies are signalling they need a rest.
On the other hand, some hens don't consume enough oyster shell. I have a couple who seem never to go near the oyster shell "bar". Their eggs are problematic.
Other hens consume plenty of calcium but still have thin shelled eggs. This happens as they get on in years. I have a few elderly hens, six and seven years old that still lay, but the eggs are so thin they break as the hen is getting off the nest. Their bodies are just getting too old to properly lay down a good shell as the egg passes through the shell gland.
Then I learned about a calcium therapy from a fellow BYCer where you administer a half tab of Caltrate, the calcium tablet for us humans. I have been doing this for the past several months when I get a hen who has laid a thin-shelled egg. I reduce the half tablet into easy-to-swallow pieces and fold them into a dab of peanut butter. It's eagerly consumed in a single bite, and the next day, the egg has a normal shell.
Caution, this is only for short periods following a thin shelled egg. It shouldn't be given as a regular thing in order to avoid overloading the kidneys.