The most common reason for this so far has been that people have tried to login using their username and password from the old website. Unfortunately you need to register for a new user account and password on this site. It doesn't take long!
On a small number of occasions we have found that someone that had previously been successful in logging on was suddenly no longer able to do so. In this case you try to login and nothing happens at all - you just keep getting the same page back. We have implemented a change to correct this and haven't seen the problem since. However - we don't know for certain that its fixed because it didn't happen very often anyway! If you do have this problem then you may need to delete your browsing history. Sorry!
If none of this helps - or if it doesn't make any sense - then please contact us for some more help.