Some hens do not process their calcium well, no matter how much they get. A lack of vitamin D could be the culprit. A complete vitamin supplement may help her process the calcium she does get.
I've been there with a hen who ate at the calcium hopper regularly and her eggs were always fragile, starting from the time she was about two years old so I can only surmise that she does not process her calcium properly.
Also, summertime and hot weather with their high need for water can cause egg issues, as well as the molt. She isn't in her big molt at a year old, of course, but if it's really hot where you are, that also can contribute to brittle shells in certain hens.