I don't know, but what other folks have said is likely correct.
I have had several bands that didn't work properly. I couldn't tell you why they didn't work, and sure you can talk to a CM if they aren't working, and that will probably fix the problem.
Alternately, if you have more than one band, you can also bring a band you already own as back-up.
Last trip, several of our bands wouldn't work to open the room door. We told the front desk, but that didn't fix it, so we just managed using the ones that worked.
We were nervous about using those bands in the parks, but they worked fine for everything in the parks. We hadn't thought to bring back up bands that trip, but probably will on our next trip.