Yes you should invest in a food that is all pellets. That way you know exactly what your chin is eating. If you read the guaranteed analysis (GA) you'll know what percentages of fiber, protein, fat etc your chin is getting. If they're only eating part of the food, they'll only get part of the nutrients.
There are lots of good foods out there and lots of bad foods. Good ones are Oxbow Chinchilla Deluxe, Mazuri, APD, Purina has a couple show rabbit foods that are good for chins. If you read around the forum, you'll find more names of good foods. Oxbow and Mazuri you can buy at some pet stores. Also Sandi on here sells Oxbow at a good price.
Some chins are picky eaters and may go through phases, but regardless you should definitely switch to an all pellet food. Maybe you'll have to try out a couple to see which one your chin likes if he's really that picky or he might just love the first one you buy.
EDIT: whoops, posted at same time as tunes, but the idea's the same.