Have you tried what Kittyfish told you? If not, really do try. I wouldn't worry about parasites, it probably is just being new to the tank. The vegetables mentioned will be eaten by most plecs, so definatly worth trying. Soaking food in garlic also weakens/kills off parasites, rather than just being an appetite enhancement
When adding veggies, try adding it in more than 1 place in the tank. It will be slightly messier when cleaning up and all, but if it is just territorial problems, it should be able to get some food from one place when the other plecs are at another.