Oyster shell should be offered in a separate dish/bowl and always available. If they need it, they'll eat it; if not, they won't. It's also better for the roosters, chicks, and any non-layers as they don't need extra.
If you're feeding her flock raiser, are you providing oyster shell in a separate bowl? "All flock" type feeds are usually higher protein, less calcium.
Probiotics can sometimes help with digestive and crop issues...not saying they'll fix hers or not but worth a try.