I am sure the change from winter to spring makes a difference . my head just turned a year. Thank goodness none tried eating those shell less eggs. They have everything they need. Just wish I knew which one was doing it but they all look alike lol.
One of my hens are layinging eggs with no hard shell . they always have oyster shells in front of them and clean water and food. And appear healthy and happy. I always find it not in the nest but rather outside the box somewhere broken but none of my hens have tried eating it thank goodness...
Try taking a sample of their droppings to vet and have it analyzed. That is the best way to know if their is some thing going on. If you are worried the rooster may be hurting them sepetate him or the last two hens from him for awhile and see how it goes.