It seems a bit odd to me that you get so many shell-less eggs. I know you give them oyster shell, but do you know they are eating it? Maybe mix a little oyster shell in with their treats or their food? Maybe give them some egg shell? Hopefully someone with more experience with soft eggs has better advice.
They have free choice oyster shell, but they don't seem to be eating much, if any of it. They are eating what I think is a quality layer feed, Nutrena Naturewise Layer Pellets. I've given them crushed hard boiled eggs, and they eat most of the shells with that.
To get them to eat the oyster shells I've found that if I mix oyster and scratch together, they'll eat most of the mix. If I just throw down the oyster shells they're not interested.
I kind of think it may just be their genetics, combined with being new layers, and being sensitive to stressors. I think they were bred to be high-output layers, and their immature bodies can't handle the number of eggs coming out.
They're all healthy and active, in my opinion. I don't see them scratching themselves or picking as if they are infested with mites, but haven't checked up close yet, and I should. I just got a 10 pound bag of 99.5% sulfur powder I'll treat them with if they're infested.
The amount of "treats" they get is minimal, and they only get maybe an hour or two a day to forage, so I don't think their nutrition is being diluted from not eating enough layer pellets.
I guess I'll just make sure to give them the scratch/oyster shell mix every day for extra calcium, and just wait it out. I'm hoping they'll do better as they mature.