For a good few months now I've had my credit card payments for online shops at Morrisons refused when they'd been accepted before. Then TfL stopped accepting it for repeat payments for travelcards etc. I've had the odd payment refuse to go through elsewhere, and now I've just lost out on theatre tickets because the card was refused for no reason.

I'm nowhere near my credit limit (never am) or card expiry date, so what's going on?

Don't tell me it's something at their end, as it's happened to too many different places using different payment systems; the credit card is the only common thread.