Here's something to consider. You said you're giving the snake 12 hour days. Do you know how long it's days were before you got it? Probably not, but if it was getting 8-10 hours of darkness and now it's getting 12... well, you get the idea. The snake would think that it's time to brumate soon. Those people that say light intensity and duration has no affect on garters are mistaken. Once the snake gets the idea that it's time to brumate because of day length clues aren't going to respond to temperature increase alone. You must increase the day length too.

Of course, we don't know why your snake isn't eating and so don't take this as advice as to what you should do. It's just one possible reason for his not eating.

What are you using for lighting?