Mine eats flake as does my ochre-striped cardinal. Start with small meaty foods. (I'm surprised to read that yours aren't taking mysis.) I started with brine shrimp and plankton. I also feed squid on occassion as well as other frozen foods like Emerald Entree. After a week they would eat whatever I put in the tank.
If you bought them at the LFS
, make sure to see whatever you buy eat first. Out of curiousity, were your thin to begin with? When I got my ochre-striped, he was one of three. The first one to go was thin from the get go and never ate. The second one never ate enough to keep weight and died about a week later. I think the trick is to get them healthy to start with, then they'll get onto whatever pretty quickly. Also, they are not naturally aggressive eaters, so you have to make sure they have the opportunity to eat before someone else snags it all. Once established and comfortable, they'll get right in the mix with everything else in the tank.