How large is she? I work part time at the National Aquarium in Baltimore and we sometimes have to entice picky eaters to eat. We throw basically everything at them. We try clams, shrimp, squid, crab meat, mackerel, smelt, krill, scallops, etc. You can get foods from the seafood section in your grocery store. Once they're eating we try to transition them to a more species-appropriate diet. But, if they're not eating, the very first priority is getting them to eat something.
Another thing to keep in mind is that sharks don't necessarily need to eat every day. Some only eat every few days in the wild. I don't know our feeding schedule for the sharks, but in the Living Seashore, we definitely have fasting days where we don't feed the animals anything. I don't know specifically how often banded cat sharks need to eat, but she might not be eating because she's not hungry. Is she at least eating something every few days?
You mention that food size isn't too big, but experiment with different sizes. The rays and skates I work with are so picky when it comes to food size. They've definitely been spoiled in captivity and now refuse to eat food that hasn't been cut to a specific size.
Maybe also try stopping the supplement for a few days. Some larger animals can be picky about additives and vitamins. If we add too much vitamin to the foods we feed the rays and skates, some of them will flat out refuse to eat it.
