Make sure you are defrosting properly - room temp water to defrost and for long enough to completely defrost the rat.
THEN use a hairdryer or (what I use) hot tap water to warm the rat up for a minute or two. Remember, you want those heat pits going bonkers but do not want to cook the rat at all. That is a big turn off for snakes.
Once warm, offer immediately, and hopefully your BP doesn't think too much about it.
Also, offer at night, and as mentioned, make sure it's cozy in her tank, etc.