I agree that boiling water is not the best way to thaw them out. Also, I'm not sure if you already do or do not, but the rat should be in a ziplock bag when you put it in the warm water. You shouldn't put a rat directly in water. My method of thawing goes like this: Remove rat from freezer the night before feeding, place in ziplock bag in refrigerator. After 24hours in the fridge the rat will be mostly thawed. I then put the rat (still inside bag) in a cup of warm water for about 30mins. When I take it out of the water and out of the bag I gently feel around the rat's insides to be sure it's heated thoroughly. I then hold it in front of my space heater for a couple seconds just for some added heat. Finally, I offer it using tongs.My BPs have not once refused a meal. I've noticed that by thawing them in the fridge for 24hrs they feel more life-like when heated up.
I also agree with all posts above regarding your husbandry!