Be sure that only one contactless device (say, your contactless card or mobile-payment-enabled phone) is near the terminal at one time.
Most contactless payments, including contactless Visa cards, Google Pay and Apple Pay, use the same NFC (Near Field Communication) technology.
Samsung Pay, however, works with both NFC technology and MST (Magnetic Secure Transition) technology, which can be used anywhere you can swipe.
You can still pay by inserting or swiping your card as usual.