Hmm, other than the ACV, maybe collecting the eggs every hour, and changing the nest lining (straw, shavings or what ever you use), every time there is a broken egg will help. They could be deficient in something.
A funny note about oyster shell. We have a neighbor that moved down here after a life of city living. He wanted to enjoy the country and he bought some goats and chickens. He needed some calcium for his chickens and someone suggested oyster shells. Several weeks later, his wife was showing me his chickens. The pen was littered with oyster shells. Whole oyster shells. He was used to parrots eating whole cuttle bone and he had no clue that a chicken couldn't use a whole oyster shell. He got quite a bit of teasing around the community about that one.
When I went out the first time today there was only one egg, that was around 10am, when I went out at 12:30 there were probably 8, so I'm thinking it wasn't long enough for miss bad chicken to get at them! We'll see how many more there are tonight!