Your Cadbury is a really beautiful girl...!!
(I am jealous; ah, ah! I don't find Barnevelders anywhere close to me.)
Where do you put the oystershell...?
If it is in a feeder, there is a good chance your hen does not eat it simply BECAUSE she won't want to eat it in a feeder...
So, I would...
Hi.
(Barnevelders are so beautiful... would you have a picture to show us?)
If your hen was mine, I would :
1 - clean her up,
2 - mix food grade Diatomaceous Earth in her feed (in a plastic feeder) - everyday,
3 - give her ACCESS to yogurt mixed (at least) with wet cat food, curcumin, ginger...