I've seen your little knife in another thread, and i have a few solutions maybe. It might be the due to tank mates in the tank. Sometimes with other domanant fish in the tank the other fish could be preasured to stay back during feeding time. Also know that clownknives along with many other knives are nocturnal. Looking at your picture he seems to be large enough for rosey reds. What i would recomend, and what i wish i did with mine is if you have an empty tank 20-39 gallon put him in it with a hiding place or two, but just him. and keep trying to have him accept pellets, once he's trained in the empty tank he should have an easy time taking them in the community tank.
If he's just alone in the tank try black worms or frozen blood worms. also try feeding him in the evening because they do prefer to feed at dusk.