From my own observations in baking audio tapes, I've found that the more tapes you bake at one time, the less effective the baking process is. It takes exponentially longer with more reels. 12 seems like a huge amount to be doing at once. Try your regular process, but cut the number of tapes in half and see if it works any better.
Of course, I defer to the real experts here, so YMMV.