Try Emerald Entree, it's a frozen cube designed for tangs, angels and butterflies. My Copperband devours it.
You could also try enticing it with a little bit of mysis soaked in garlic. I'm not a believer that garlic does anything beneficial for the fish other than entices finicky eaters to start eating. You could try Spirulina enriched Brine Shrimp as well. Also, Reef Nutrition Articpods can help. Another thing to try is shaved wild salmon. I buy wild salmon from the fish market, cut it into cubes and freeze it, and grate it on a cheese grater as needed. Salmon has some really beneficial things in it and can entice finicky fish to eat. It's very fatty, so you don't want to use it as a main stay in it's diet, but as a treat or to help it start eating, it works great. I'm sure your LFS will have frozen food designed for butterflies. They're usually designed for butterflies, tangs and angels. Just make sure it is not an herbivore only blend and it has some meat in it.