First things first - check that all the details you've used to register are correct. If you've found an error then we'll need some documents from you to update your details - have a look here.
No typos? All looks good?
In that case, it could just be that the sources we have out-of-date information which could be due to:
a recent change in address
a recent change in name (due to marriage or deed poll)
Don't worry, you'll just need to send your documents over to our lovely verification team so they can manually verify your account.