Just a note have you considered pulverizing the oyster shell to the size of sand. It might be easier for her to take it in. I never seen my own hens eat the oyster shells I have out there, but did notice the sand sizes ones were gone and they like picking off the PDZ in the coop.
I agree with your approach on what your doing with your hen, every chicken is different with almost everything including having likes and dislikes and observering there behavior to find a solution for possible problems that might occur.
My flock get there usual layer feed mixed with 18 percent feed available everday, in the mornings they get whole corn, kale, spinach, sunflower seeds, and fermented scratch. In the afternoon they get mealworms, grass clumps, and weeds from the garden. And I sometimes leave a tub of plain yogurt...
Is she at the bottom of the pecking order? Maybe she can't get to it cause the other hens will pick at her?
Do you ever give her fermented feed or plain yogurt? It helps with digestion, allowing the body to absorb maximum nutrients better.
I've been using this method for my flock and had zero...