I've had a few snakes that would do the same thing from time to time.. Two of them did this as a result of me banging around the rack while they were eating.. When I left them alone and quiet they both would go back and eat the mouse.

something else to consider; have you changed suppliers?? different mice. maybe a slightly different smell...

if you fed f/t i'd recommend washing the mice then offering them..

hope this helps some..

byron.d