I have had the exact same problem with my female bee when she was around 800g. She would only take mice and was afraid of rats. What I did was give her mice, but before i would let the mice sit infront of her tub (scenting) before i offered them to her. I tried a small rat the next feeding while scenting with mice and there was no luck. I had the problem with the rat bumping her and she would freak out into a head sping thing. So what i did was did the scenting again, then offered a weaned rat a tiny bit bigger than a mouse. She took it well. From there I worked up to a live small rat. before this she would refuse any frozen, but now she is pounding frozen small rats weekly. I think its not the mouse these bees get stuck on, its the size of the rat combined weith the wobble. Its almost as if they know that it impares there feeding respones and they have fear of the live larger rodent. Now my female be is 1100 grams and gaining for next season, looking for some killers!