I have had similar issue with pleco and cichilds where they would not eat. Usually it is a parasite issue. In every case I have treated the tanks with 2x metronidazole (use 2x the recommend amount, will not hurt anything but make sure you remove your carbon while treating and do a 25-30% water change on the 3rd day) and it has fixed the issue. Had this happen last week with my wild caught L-14, L-48 and niger cats. After 24-30 hours of treatment it was like a switch and yesterday and today they were eating like crazy.
I treat all my wild caught fish with metronidazole when I first get them. I doubt importers and most dealers are going to spend the money or time to treat their fish.