These fish are VERY sensitive to oxygen levels. Much more than any other fish I've ever kept. I killed two of them last year when I didn't notice that the spray bar on my tank had slid down the glass an inch or so. Combined with a medicine that used oxygen, this killed two Sexfaciatus and nearly killed the unkillable Lussoso.
Any time the surface distortion starts to slow a bit due to reduced flow or something, I can always tell because the distichodus starts breathing really hard.
Otherwise they're pretty hardy and not picky about food or even nitrates. I don't know what else could be causing your issue. But keep an eye on the oxygen exchange.