Maybe it's not thawed to a warm-enough temperature? I know that someone (I think it was Smulkin) suggested warming the thawed rodent with a heatlamp for a few seconds befpre feeding, so the prey gives off a better heat signal to the snake.
I know that one of My Bps, Goblin, will not touch f/t unless it's warmed pretty hot. You just don't want to cook the rat, though, which will cause chemical changes and lessen the nutritional value.