I've never seen a copperband eat flakes or nori. I'm sure they exist though.
Getting a copperband to eat is a common challenge. Many people have to start with live foods like live brine shrimp and/or live white or black worms and then try to transition them to frozen foods like fozen mysis. Some have siccess feeding them clams on a half shell. You have to try a lot of thigs, otherwise the copperband will starve.
Best thing will be first to ask the LFS what it was eating. It's always good to SEE it eat at the LFS first if you can.
I would buy a selection of frozen foods and start trying those. Is the copperband timid kr will it come up to the glass when you are there?
My copperband eats well, but it ONLY eats directly from my feeder pipette one piece of mysis or one worm at a time. Of they come out into the water column, they just float away ignored. She has to grab it out of the tip of the pipette.
Challenging, but a worthy fish!