soft shelled eggs

Am sure that this is not a new problem  but 1 of my 2 hens has begun to lay soft shelled eggs , she is worme d f,ed, treated  the same as her "sister" but has  for the last few weeks laid soft shelled eggs.I have added cod liver oil to her feed over last 2 weeks and have ground the oyster shell down in case it was too big  for her but what next ?