I'm not really a colubrid guy, can you hook train a corn snake? lol. The old feed outside the cage idea was a good train of thought, but it's also a very old train of thought. Yes, snakes are creatures of habit, but it's not good for them to be moved after eating. Also, they're learning that they come OUT of the cage to get food. Not to mention with something larger like a burm or a retic can be very dangerous to move after feeding as some of them stay in "feed mode" for days. What we do with our larger ones is condition them to only being fed on days they don't see the hook. If they see the hook, they know no food is coming in today.
What else has changed? maybe a slight RI or something that could be stressing her out? been in open view for awhile?