When I first started into the hobby clown gobies, and citron gobies were all considered 'easy' fish. I had two yellow clown gobies, and they both ate like pigs from the beginning, were fearless, and had great personalities. When I moved I lost nearly all my fish, including the clown goby. When I started restocking it was on the top of my list, and over about 8 months I tried six of them. Different colors, different stores, made sure they ate in the store, etc... They all got in the tanks, and ate, and ate, but wasted away. The only thing I could think of was that they were being collected with cyanide, so they're digestive systems were wrecked even though they looked healthy, and ate.
Like the others have said, you can try soaking the food in garlic which will sometimes help get a finicky fish to eat, or try some live brine if one of your LFS sells it. This will often encourage a fish to eat if they're just stressed out, but otherwise healthy.
Good luck.
B