I've had the same (or a very similar problem) twice in the last month. I call customer services and they take the block off. Apparently it is being activated by Mastercard. The problem is, I am not being told it has happened. So it takes me several transactions to work out there is an issue and to phone customer services. This CANNOT be right. I'm sure both Mastercard and Tesco feel I should be told my card is blocked, so that we can sort it out. I've had to switch to my Visa card, which I'm sure neither MC or Tesco want. Frustratingly customer services said there was nothing they could do, and they could not flag it as a problem or raise an issue! So I'm trying on here.