I wouldn't consider force/assist feeding until you look at the snake and think 'That animal is about to starve to death'. It's incredibly stressful and generally a bad idea unless you're talking about babies.

When mine wouldn't eat I had it recommended to get a paper bag and a rubberband to shut the top of it with, heat the mouse up and put the snake and mouse in the bag and leave them for awhile. I never had any luck; ended up having to feed live before mine would eat, but it might be worth a try.