Some plecs don't adjust well - I recently lost a gold nugget (the store also lost most of the shipment) who looked perfectly healthy, but starved to death. Sometimes there's nothing you can do, they lose the gut bacteria they need to digest food as a result of shipping stress and can't recover. Some plecs also come in with internal parasites or worms - if your plec has a hollow belly, I'd remove him to a QT and start treating with metronidazole. Try feeding frozen brine shrimp or bloodworms thawed with some crushed garlic, it can help stimulate sick fish to eat. This worked on my new gold nugget, he's from the same shipment as the first one, but has settled in and is now eating well.