If it hasn't been mentioned already I would avoid boiling water. Any risk of cooking the rat at all is best avoided. I use a cheap hair dryer on its low setting. It's quick, easy, and gets the scent in the air which usually gets the snakes interested quickly.

In my experience the inability to 'find' the rat that you describe is pretty normal. Sometimes they'll take it on their own no problem, and sometimes it's like they just don't recognize the food, but they know it smells like food.

I keep the rat at around ground level and shuffle it around a bit. Usually they take it quickly, if they don't I might try shuffling it a bit closer to them, or shuffling it a bit farther away. They seem to go for it when it gets close or farther from them. Sometimes they'll constrict but wont swallow it, and they'll go poking around looking for it like they've somehow lost it after constricting and then give up, in which case I retrieve it and offer it a second time.

It gets a bit frustrating sometimes, but it's always worked given enough patience.

Trick is just to be patient and to remember that they can be a bit skittish, even with prey.