The batch of f/t rats that I bought recently had a very strong ammonia and chemical smell to them... my male, Zyali, won't have anything to do with them. I'm considering washing one next week before I offer it, to see if that changes anything. I've never used Co2, so I don't know if that leaves some kind of odor on them, but he's never had an issue with taking mice and rats from the same source before. I've never noticed the chemical smell on them before, though. Do the ones that your snake won't eat have a chemical smell?