I think there are several possible causes for the meal refusal.
1) As others mentioned, you may need to clutter up that enclosure to make the snake feel more secure. Also, no handling until it is eating for you, and make sure your temps are correct.
2) 90F may not be hot enough, normal body temperature for rats/mice is more like 99F.
3) You are taking a snake that may still be stressed from the change in environment and trying to switch from live prey to f/t. You may have better luck getting it to eat for you by feeding it what it is used to, and then making the switch after a few meals.