That's odd, but before exchanging...
-does it work without a case? I ask BC you didn't specify
-my local Best Buy has a demo area to play with Samsung Pay, if convenient it might be worth a trip to see if you can get it to work there.
I ask BC I notice that with certain machines I have to really try to get it to read, sometimes it takes a few attempts to get Samsung Pay to work, specifically in Target. I don't know if it is BC the card reader is deeper or what, but I would try it out a little bit more first.
But if not hope your exchange goes smoothly. Oh, and mine works fine with a Tech 21 case.