My iPhone 6S Plus won't work on Apple Pay contactless readers.

I had an iPhone 6 that worked fine with Apple Pay using my Citibank United Explorer Card.

I recently purchased an iPhone 6S Plus (iOS 10.1.1) and added the same card. When I go to pay I put my iPhone up to the scanner, tap Apple Pay and my fingerprint scans as always. Everything seems like it is going okay, but the check and "Done" never happens. It just hangs there and doesn't complete the transaction. This happens at both Whole Foods and my local coffee store. Same issue at many different registers at both stores.

So here's what I tried to do, to no avail:

1. Restart iPhone.

2. Removed Citibank United Explorer Card from Apply Pay. Reinstalled it. It reinstalled fine, but same result at the reader.

3. Put another card on my Apple Pay. Same result as #2.

4. Put my Barclaycard on Apple Pay (thinking maybe it's Citibank, and that the card most closely associated with Apple would certainly work). Same result as #2 and #3, card installed on my phone no problem, but wouldn't complete the transaction at the register reader.

Why did this work fine on my iPhone 6 and not work on the 6S? It is very embarrassing that this doesn't work.

Thanks!