Add garlic to the food. Try fresh seafood from your grocer. Clams, Mussels, Scallops. Also you could try frozen from the pet store: krill, silver sides..
Sometimes Pork Puffers will go on a hunger strike. I've also heard of some Pork Puffers "turning off", which consists of them starving themselves and dying. Lets hope this situation is not the later.
Soak food in Garlic and post updates.