There could be a number of reasons for why your key/card isn’t working, which is why it’s really tricky to give a blanket answer that applies to all situations. The first thing to do would be to wipe your key/card to make sure that it’s clean and there’s no dirt interfering.
If this doesn't help, note down the error code that’s appearing on your meter and post it in the comments below. One of our other users might be able to help, but if not, the Moderators will be able to step in to offer their assistance.
If you need a replacement key or card, the best thing to do is contact our team on 0330 102 7517 and they'll be able to arrange this for you.
*Updated 19/07/2019 - All info correct and up to date*