Maybe try a smaller rat.

A few of my larger girls have missed a strike on a rat in the past and it has startled them a bit on larger rats. These two girls will shy away from a 60-80g rat, but take two 40g rats without hesitation given about an hour or so between the two smaller rats.

My ferocious Pastel stuck at a medium rat that charged back at him. My pastel laid there like he was dead for a minute. That was a month ago, and he hasn't taken anything since. They sure are finicky critters!!