Make sure the temp of the prey item is close to live prey; around a 100 degrees or so for a live mouse/rat. Another thing you could try is braining one. Kind of gross but sometimes splitting the skull will let a little blood out and that can excite them enough to strike and eat it.

Mostly it comes down to patience and perseverance. You might have to alternate every other feeding with a live and then trying a f/t the next time. You could also ask a pet store for some soiled mouse bedding from their live mice. Rub that on the head of the f/t and it might trick her enough by smelling like the real thing that she'll take it. If she's a young one, you don't want her going on a feeding strike and losing too much weight if she's still little.

Either way, good luck!