I would heat up the head of the F/T prey a little more.

I may be wrong but I think when their mouth is open their eyes are obscured and they rely on the heat pits to home in on the prey.

Of course some of them do just spring out and wave their open jaws about wildly - I call this the "is the snake keeper paying attention" manoeuvre.

It is responsible for most of the bites I receive while flailing the rat desperately at them and pooping myself.