Only problem with that is that B-12 has never, in any scientific tests, proven to be an appetite stimulant, in spite of its being used that way as a folk remedy even by some physicians.
http://pediatrics.aappublications.or...tract/21/5/860 <--reference.
There are also people who will tell you that metronidazole (Flagyl) is an appetite stimulant. It's not, either.
Things I would try (and I have to assume have already been tried, after all the folks talking here are reasonably experienced): A larger bin. A smaller bin. Filling the bin with crushed newspaper. Move from the bottom of the rack to the top, or vice versa--or from left to right or vice versa.
Move to an area with less traffic.
Adjusting the feeding cycle by a few days in either direction.
Adding an appropriate-sized hide box, and feeding at night, in the dark, when all is quiet, with as little disturbance as possible.
Testing for (and treatment of, if necessary) internal parasites. I would consider preventive treatment even if none are found, as a last resort, as some species shed oocysts or eggs only intermittantly and thus are hard to detect.
But B-12? Why? No logic behind that. These animals eat whole prey. They aren't too terribly likely to become deficient in vitamin B-12.
I realize that you feel it's worked for you, and you've said so, but insisting on it and implying that folks who don't take your advice are 'letting the animals suffer' is just going to get you whacked with a great big stick labeled 'scientific method'. Put it to the test in more controlled circumstances, and I bet you will find...just like everyone else who has ever done so...that there isn't much to it after all.