If all else fails try a gerbil. In my experience it never fails. It's been awhile but it's been speculated that they are naturally inclined to them. In the wild balls eat something called jerboas, which is not a gerbil but it's been suggested this might be why. I've had to do it a few winter times with a couple balls and it worked every time until they were willing to take rats again. I'm a big proponent of this method.

Seems also a lot of people have success with braining rats or mice. Puncture the brain before presenting it.