So she's not constricting right away, she moves it around and then eats it? When you up the size of the mice to at least hoppers, the extra size may cause her to constrict.

Also, it almost sounds like she might be used to the F/T food and understands that there is no need to "kill" the food because it is already dead. I have heard of snakes that have been fed F/T most of their lives that will just go up to the rodent and begin swallowing without any striking or constricting.