A lot of good suggestions, earthworms are a perennial bichir favorite, nitrates could be too high(the tank IS cycled isn't it?), check temperature, I've had fish that wouldn't eat below 74F.
If feeding pellets and they aren't known pellet eaters, you can try rolling the pellets in fresh crushed garlic(It's in a lot of pellets already as an appetite stimulant and attractant, but fresh always works best!) Once they recognize the pellets as food, you can quit the fresh garlic. Hard part is getting them to recognize them as food.
If they're tiny still, you may have to chop up any worms.
That's everything I can think of off of the top of my head without more information than they're bichirs that aren't eating.