I weigh Quetzal weekly, two days after his food day. I'll also weigh him right after he poops to compare "empty" weight. If there's significant drops in his weight, I know about it right away.
I've got a classmate who's BP will ONLY eat live female albino mice at night in complete darkness and silence... otherwise she just ignores her food and lets it wander. Try everything: at night, in silence, in cage, fresh prekill, live, f/t zombie mouse dance, males, females, colors, etc. Rats and mice don't work? Start trying ASFs, gerbils, and hamsters.