Most recorders only record about 24 hours of video before they recycle.  The really robust ones can keep about a week at once, but that's it.  They're designed to help the banks out when you call and say that you just got held up--not a month later when police finally realize that a given ATM is ripping someone off.  (In fact, early ATMs with cameras took a still photo, and only then while you withdrew cash--totally useless in this scenario.)