I know someone who recommends giving your thin shelled girls calcium drops. Assuming you free feed oyster shell and that is not enough.
Some hens don't process the calcium well, no matter what form it's in, unfortunately. It's also related to vitamin D as well, so if her body doesn't have them balanced as they should be, it can result in thin shells. This has been something since she has gotten up in age, so I believe that has a lot to do with it in her case. Trust me, they have plenty of calcium, always have had. I've seen this in one other hen I have, so it's not the calcium, per se, but something with the way they process what they do eat.
. I have a few girls that seem to be having issues this season im gonna try it with. Does not mean it would work for your girls but might be worth changing stuff up to see. Or not as the case may be.
What about the Snuffleluffagus?