40% is on the low end of appropriate humidity. You could try bumping that up to 50 and see how it does.

Also, what kind of hides are you using? One of my snakes prefers half logs (he won't go in anything totally closed in) and my other snake is stressed like crazy with half logs so she gets the little rock-looking enclosures that has just one exit. Once I figured out the hide preferences, it seemed to bring the stress level way down.

I would make any changes you need to make, and then leave everything alone for a week before attempting another feeding. When you do feed, try it in the enclosure. Like someone else said, sometimes a blanket over the tank helps them to feel even more secure and may aid in the feeding process.