I have sometimes had problems using credit cards in Europe especially since when you get out of the major cities in some countries (as happened to me in Saint-Malo France a few years ago). I learned then that if your card doesn't have a chip (most American cards do not, though MR Premier cards now do) it's harder. I desperately wanted to buy a painting but the guy selling it couldn't get my credit card to work. He finally took it to a restaurant and they told him to sweep it through faster. He did and it worked. One lesson learned.
Before I left for Venice, I heard on CNBC that Europe is officially again (or still) in recession). That may explain some things. Places in Venice where I used to use my American Express card (even in Piazza San Marco) no longer take it. Fortunately they took my Chase Sapphire Preferred. But I think that while Amex was always less likely to be accepted in Europe because of its higher percentage of the take, now it's worse. It'll be fine in restaurants if they display it on the door, but make sure you have another card in stores unless it states otherwise.
Then I had yet another new problem, partly because of the design of the newer cards. Both my Chase Sapphire Preferred MC and my MRI Premier VISA no longer have raised numbers so when I went to get a refund in the usual place in San Marco, they couldn't process the cards because of lack of raised numbers! But the good thing is they COULD process my Amex card for my refund because it did have raised numbers.
So there are some new things going on, I expect mostly because of the recession. I will write more about Venice -- which I love more every day, even now that I've experienced acqua alta -- but I think this is likely to be a trend elsewhere.