Well I would wonder if the other fish you have are eating the food before the clown can get to it as they are a bit more aggressive, but you say it is eating a ton so that's not the case. I know when I had a pair of clowns they were somewhat timid about going after food I had put in the tank and the other fish would usually grab it. I finally started feeding with a turkey baster so I could drop the food right in front of the clowns at their anemone. I would say as long as it is eating regularly and doesn't look sick in any way, not to worry too much about it. You can try what Lonestar suggested by adding another clown. If it doesn't fatten up it will at least have another fish of it's own kind to hang out with.