I've got a recently introduced copperband here who was in pretty much the same state; picking at the rocks, ignoring frozen foods.
As an enrichment for my tangs / angels / klein's butterfly, I've been using a veggie-clip on a string to hang either a frozen/thawed littleneck clam or blue mussel (I get them in the seafood department of the local grocery store) in the water column. The fish like to pick at it as it opens over the course of the day. Well, it turns out that shell, open by about 1/4", is a perfect spot for a copperband to slip his nose into and pick out mouthfuls of mussel. It does mean hanging a mussel in the tank every day, but so far, so good - and hopefully, s/he'll realize the prepared foods that I use are also pretty tasty.
*I can't guarantee this will work for every copperband - this is at least the third one I've tried.
~B.