Hmm how big is your snake? A small rat is a huge difference between an adult mouse. Some snakes like smaller meals and are intimidated by larger ones.

Also there are many ethical issues with have been debated millions of times. You can find any of these threads by doing search "Feeding" and I am sure you will find a debate. I never feed rats, but what from what I have heard and someone please correct me if I am wrong, but small rats teeth can't penetrate snake skin yet. So knocking it out was pointless.

Your snake also might not like p/k 'd food. What was he eating before?