Some adults are very difficult to switch over no matter how long you've had it and/or how healthy it is. My advice is to give it an extra week or two between feedings. My adult jungles will go up to several months without food during the breeding season....this is where the "healthy" and "patience" parts really come in. As long as your snake is healthy and you're patient enough to walk away for another week after a refused meal, you shouldn't have any major problems.
Keep scenting in a ziplock, but rinse the rat first. I've washed them in the past, but lately I've noticed that washing makes them harder to get used to the rat scent. Here's a couple pics of my "washed rat eater"...at least now he's taking them rinsed!


-----
"He who would stifle debate rather than engage in it, does so at the expense of his integrity and credibility"
Mike Curtin