How are you defrosting and offering the prey?
Properly defrosting the prey and heating it properly before offering to a BP is important to have them recognize the prey as food.
Any questions on defrosting and heating prior to offering, just ask.
I agree that you should be offering F/T mice first and then attempt to switch to F/T rats down the road. Too much all at once for most BP's to switch to F/T and rats.
It sounds like you are doing it correctly, but thought I'd ask. I'd defrost in room temp water for a while - making sure it's thawed all the way through, and then quickly warm up without cooking it at all. That means a quick hit with a hair dryer or warm water for 30 seconds +/-.