I was in the same predicament over year ago but 'nothing' seemed to work in the conversion of mice to rats until I started using 'dirty mouse bedding' and tough love. At least w/ dirty mouse bedding, you don't have to worry about wasting a new mouse each time. Just a thought....Good luck!
Just how long do I hold out? He has refused 3 rat pups in a row and it's starting to stress me out!