If she's not taking the oyster shell - and my girls don't like oyster shell, either - crumble up the shells of the birds who are laying solid eggs and give them back to them. Also give her yogurt and cottage cheese as a way to up her calcium intake and see if that helps.
Do you have any idea how old she is or how long she's been laying? My girls laid all sorts of thin shelled, "rubber" shelled, and no-shelled eggs when they first began laying. Took them a few weeks to figure it out but now they're laying fine.