Are they in vivs or tubs?
What did the breeder have them in?
Do they have hides?
The Q's could be endless

I got a little fella in Feb. 227g, he went off feed for 6 weeks, with a weight drop, I finally figured, he should go into a smaller tub, he hasn't missed a meal since.

Just make sure your husbandry is spot-on and they'll come around and feed