In this period of the year garter snakes (and other reptiles too) tend to stop eating or they eat less than normal.
This is caused, amongst others, by the sorter days (influence from the outside through your window) and it is getting colder. The last the snakes feel the best during the night.
Their instinct tells them it's getting tome to think of hibernation. If you don't want them to hibernate yet, you have to make sure the temp goes up (day and night).
There is ofcourse always a possibillity that your ske is ill, but I cannot judge on that from beghind my computer in Holland
