Bump up to a .22lr? Probably not a particularly helpful suggestion.
My guess is you're having marginal hits due to them moving and it won't matter what pellet you use. A rat is large enough that you still have to hit it in the right spot to have a quick kill.
Also- does it matter if they aren't DRT? Get a pellet in them and hope they die somewhere along the line. Unless you start smelling and finding dead rats everywhere.
-Jake