Hey Jen, welcome to the forum. As far as your checkered not eating....there could be a ton of reasons why. What temp. are you keeping him at? Too cool temps. will slow them down and they usually won't feed.....too high temps. can also stress them with the same result. Has anything else changed with him? Less activity, change in caging or where the cage is set? Have you tried giving him other prey items, like fish? Sometimes the flopping movement of a fish will get a problematic feeder "in the mood". I'd use rosies or platies, not goldfish. For the most part, as long as he's got good weight, is active and has no other outward signs of any problems....snakes can go quite a while without eating. Three weeks is kind of long though for a garter. Hope this helps some.
Roy