It is common for puffers to go on a periodic feeding strike. Have you, by chance, noticed any stringy white feces? These animals do get bored of food and their environment quickly so you will need to feed a variety of foods (silversides, capelin, and various
sw fish, shrimp w/shell on, squid heads, etc) make cookie-cubes, and/or use shells or other novelties to keep the fish interested.