I feed mine crushed flake, crushed New Life pellets, Hikari micro pellets, frozen brine shrimp.
They seem happy to eat anything small enough, they have never been picky.
I do find them to be particularly susceptible to disease though
When I first got mine, I put them in a bare bottom qt tank. They were quite skittish, and didn't want to eat at first, but would pick the food off the bottom at night when they felt more secure. A few days of that, and they got excited about feeding time.
I don't know if that is relevant or useful info for you, but it's all I got
Up till today, they were in a bare bottom 10 gal QT tank. I have a 40 breeder cycling so I was hoping to leave them in the 10 gal tank, but I have a sick fish that needed QT. So I moved the Threadfins into an established, but unscaped tank. There are a few plants but nothing major. I'm hoping the larger tank with a few plants may stimulate their appetite.
it may be a water issue. if fish dont act normally and you've varied everything else, it probably environmental conditions. i'd check your water parameters. perhaps all that crushed uneaten food is doing a number on the quality of water.
you may want to invest some time in culturing some live baby brine or daphnia until they'll accept something else. baby brine is a great food all around anyways and would be good to feed any fish. adult brine not so much.
Mine couldn't eat the hikari micro pellets at first... I fed them crushed up flake food and hikari's frozen daphnia (smaller than the other brands I have tried). I also buy 20% more fish than I want to end up with... seems like some always just waste away.
I fed mine pulverized Spectrum pellets and was unsure during the whole time that I had them if they were eating them. Mine bread and produced fry! All the fry but one were eaten by a trifasciata that I'd put in their tank (gave up on breeding them about the same time they decided to breed). The one fry lasted about 4 weeks = (