Let's hope there has been no damage caused by the copper treatment. So many times I see fish, especially certain species, and puffers being one of them, not handle exposure to copper well. Even when the fish seemingly handle the treatment well, they decline and stop eating after a while, sometimes as much as a year later. If this is the case there is nothing you can do.
Another cause could be a change in tank dynamics, any angst with another mate.
Clearing parasites from these guys is also a difficult task, being since last September I wouldn't lean that way. But that could also explain the loss of appetite. As far a treatment goes, prazi alone is not good enough. There needs to be metro in the treatment as well, general cure contains both prazi and metro.