Make sure the husbandry is spot on. Mimic the natural environment as best you can, with appropriate temperature range, humidity and hide spots. Offer prey similar to what this species may take in the wild. Some young rattlesnakes are lizard eaters by preference, though most will accept pinkies or fuzzies.
Once you are sure the environment is bang-on correct for the species, if the snake still won't eat, it's a good bet that it has a problem with parasites. Doing a fecal check is a good idea. Some keepers just shotgun with Panacur and Flagyl, which are pretty good standbys.
My web page has a lot of articles on how to medicate and assist feed venomous species.
http://www.snakegetters.com