EmmaDonovan
Consider the fact, that anything the chickens eat,,, aside from starter crumbles,,, and Brocken up bread,,, they need grit. I know lettuce is soft,,, but it still needs to be ground up in gizzard, to digest well. Consider getting chick, or chicken grit.
My chickens are all grown,, so They pick up their own grit from the ground. I do offer them Cracked oyster shells free-choice. That does work as a grit also, to a certain extent.
Once your chickens are somewhat grown,,, and inside your run area,,,, If there is grit size gravel pieces in your soil, then chickens will find their own choice grit pieces.