Just a generic rubber case I got at a mall kiosk for $13 - basically it's a placeholder case so I at least have something protecting the phone until I can get something better.
I'm going to try it again the next time I pass by the ad and see what happens.
Could just be the ad as well. If it is outside and uncovered, I don't know how durable those NFC tags are to the elements. You could always just take the case off for a sec to experiment if it is the case or not