Think of the smaller feeder animal not as a real meal and more as an appetizer (literally) - something to get her back into the swing of things. You mention that she was taking live small rats from her previous owner. If she has had any incidents with live feeders, a poorly placed strike and resulting bite or scratch for instance, then she may be more reluctant to take another live rat. Taking a step back and offering smaller (less threatening) prey animals can help rebuild confidence and feeding response.
Exactly what i ment by it just not in as much detail