I am having a similar issue,
I bought a juvenile clown from the lfs to pair with my larger female. They get along great but the issue seems to be the little guys appetite. I've fed frozen brine, freeze dried, and hikari and formula one pellets. Nothing sparks his interest. He just hides in the very back of the tank all day. I dont believe he is ill because he seems to be swimming normally, but I did noticed some small nibbles out of his tail. Could this be it?
He has only been in my tank for a few days so maybe he's still shy of his tank mates, but it has be worried.