I've had several problems. I was at Target, paying at the pharmacy. I used Samsung Pay and it worked, then I had a second transaction at the pharmacy that was declined. I was also declined at the main registers. I took the same card out of my wallet and used it successfully.
I have tried three different times at HEB, at three different registers. Each time the terminal accepts the card data, asks me 'debit or credit', I press credit, then the 'transaction is not approved'. HEB has NFC and chip reader terminals, but those parts aren't activated. I was also able to use the same card to pay when swiped.
I have used Samsung Pay successfully at other places. Randalls/Safeway, Target (on previous trips), Kroger, local restaurants, bars, and convenience stores. A lot of their equipment was really old, but I had no problems. Of course it doesn't work at gas pumps or on the USPS automated postal machines, because they have the card insert slots.
I did have the problem that others have reported with my accounts being flagged for fraud after I added them, but a phone call took care of that.