Out of my collection, I have two that I've called picky eaters in the past.

One is my western hognose, and the other is my BP. The Hoggie used to oblige me almost perfectly every other week. She'd never eat while I was around either, never ever off the tongs. Then all of a sudden I tried running the rats under hot water right before feeding, and she's taken them immediately ever since.

My BP's a sissy. He never strikes. My current reliable method for him is to wait until he's in his warm hide, run the rat under hot water, then stuff if in the entrance of his hide so he can't get out without pushing it out of the way. It's usually gone within an hour now.

Nothing I hate more than wasting rats, but I've been really lucky, I haven't had to throw any out in quite a while now.