The one food I've always had luck with when the puffers wouldn't eat was frozen, raw squid. Small chunks at a time. It is not uncommon for a puffer to not eat for up to a week when it has been moved. They stress easy. The best solution is to make sure it has plenty of hiding places, is not being harrassed by other fish in the tank (yes, many of them are big chickens until they settle in).
What kind of puffer is it? Frozen formula 1 cubes are also something to try. Most puffers love them, and there is garlic in it to help stimulate appetite and strengthen immune system. Be diligent, keep trying. Try also shortening the length of time the light is on over the tank for a few days. This will also help to reduce stress. Try to avoid scaring the fish, fast movements around the tank, fast movements when trying to feed it, and anything else that might startle it. When stressed, they will refuse food and literally starve themselves to death.
Good Luck and let me know if you need more options... I have others once I know what kind of puffer it is.