hrm.. I can see how it could be several of these factors then.. On two occaissions of buying the rat I was not able to straightway go home and freeze it, it had 30 to 60 minutes in room temp.
I probably have been using too hot of water.. I microwave my water to get it nearly boiling, put the rat in a ziploc and set it in the water. I was doing that straight from the freezer.

I purchase from the local pet store [jacks aquarium and pets for me]. I will take them straight home, put them in tough freezer bags and into the freezer immediately. I will also try letting them thaw overnight in the fridge a day ahead of time.

as for the heating part.... I guess I'l try less hot water it just might take longer. I don't currently have a hair dryer. crazy huh? I am a girl with long hair and I don't use them.