I think that stunning is bad too. not only for the ordent but oyur snake, the rodent may come too in the middle of feeding, and well being eaten alive isn't fun I am sure. Have you tried buying frozen thawed and putting them in chicken broth? Or, opening the skull to expose the brain? these may work. If it is a case of changing form Mice to Rats, then rubbing (really rubbing) a mouse on a rat usually works for me, even cutting some mice hair and placing it on the head of the rat. Another reason, is maybe your food isn't warm enough... heat it up more, and whe nyou dangle it from tongs, well tap it near the heat pits and the tip of the nose, usually this makes the 'strike' happen, and they swallow from there... JMO
If toast always lands butter-side down, and cats always land on their feet, what happens if you strap toast on the back of a cat and drop it?"