Sounds like it couldn't hurt to facilitate a nice sharp drop in temperature of about 10 degrees and perhaps a mist at "sundown" or first thing in the morning.

Often times it is not the food being offered that is the problem. Often it is the timing and other factors that make the snake hesitate. Personally, I will refuse food for the first few hours of the day, then eat heartily after 2-4 hours of activity. If "my keeper" was only offering food at that wrong time...

Most king snakes and milk snakes will refuse food during the summer day but eat that same food enthusiastically at twilight or night. I don't know about dekayi but most day-active garters I have will be more "ready" to eat about mid-morning or shortly after their morning warm-up and have just reached optimal temperature. Enthusiasm can wain outside of that time. If there's not much difference in temperature from sun-up t mid-morning, would the time ever be right for eating? Give your snake not only visual cues, but temperature and humidity cues daily so they get that rythm and always know day from night. Full spectrum low-intensity lighting and a drop at night/warmup in morning doesn't hurt. It makes a big difference in garters appetite and activity levels. Maybe in dekayi too.