I have noticed other than at CVS, American Express always seems to manage to try to escape a store's block of NFC. Have you tried the same Amex in Apple Pay?
I don't have an iPhone 6.
I could try Google Wallet at Michael's but they're not the friendliest crowd here and since there are no logos to point to, they might react poorly.
And no, my Amex works FEWER places than Google Wallet. Google Wallet works great at Staples and Walgreens, whereas the Amex card is rejected (may be related to the service code on the card). The service code on this card is 702 (Closed loop, goods and services only) and I wonder if the "7" closed loop flag causes rejects at Walgreens/Staples.