if you cant see any sign of ich, then possibly some sort of flukes. fish can have these parasites without any other real signs of a problem. I have gourami's that were scatching for no apparent reason, we didnt know what it was until one got fungus. It was then suggested to me on here that it was probably a parasite that had caused the initial damage to let the fungus set in.
probably a silly question but have you tested your water to be sure it isn't an irritation caused by highh amonia, nitrite or nitrate?