Phone vibrates but no transaction occurs

I have successfully used Apple Pay with my current iPhone (6S), but recently I've been running into the problem that when I touch my iPhone to the terminal device the iPhone vibrates, like it has done its thing, but the terminal does nothing. No transaction. My iPhone seems to be doing its part, but the terminal does not respond. I get no messages, errors or otherwise. Just left at the register, sheepishly pulling out my wallet. It's happened three times in a row at different stores, and different terminals. Any thoughts?