NFC working intermittently

I have an iPhone 6s with my credit card stored in the Wallet, recently it has been working intermittently. I took it to local Genius Bar and they did a software update and it appeared ok, until the next day when it failed again. However later that day it started working again. They had checked the NFC in store and it was ok. Since then I have reset to a new phone and restored the software, and it still has the same problem. The restore was from a backup done six months ago when everything was good. I think it must be an intermittent fault with the NFC. Any thoughts please.