Since you were able to add your card, I'll assume that you are not rooted or hid root until you added the card. The old Google Wallet worked just perfect for me so I know that my NFC device was working along with the authentication module. I forced my upgrade of Google Play Services and Android Pay. Three days ago when I was at McDonalds none of my cards would work. I even tried to hide root and nothing worked.
I removed the Google Wallet application and reinstalled the now posted Android Pay app. Yesterday at Staples and Wallgreens it worked perfectly. Based on my experiences, I would suggest checking to make sure that you are on some version of 8.1 or later of Google Play services. Uninstall and reinstall the latest Google Pay app then try it again.
The antenna is located on the back cover below the on/off/volume rocker switches all the way to the speaker. I have found that you do need to make sure that you are in close proximity longitudinally and latitudinally to the antenna distance from the other device can be several millimeters. Mine works great through a case.
I hope that this information helps.