Could possibly be a temp issue. Are you sure they are thawed good? Are they too warm? Was his response better on live? This is the reason i feed live. Im not turning this into live versus frozen. I always seem to have good results with live cause if they dont eat the rat isnt wasted. But switching over from mice to rats can be a chore. Just be patient and keep trying. I wouldnt scent it to mice just cause then your gonna have to keep doing that. I would be persistent. Eventually the switch will happen. Just keep in mind they can go a while without food so just stay the course