Must say I've never failed to join a race when I've signed up, but then I'm 99% sure I always sign up on the mobile app or Zwift Power before I log on to the Zwift SIM. This may (or may not) support this theory.That must have been it, I was mid way through the login in to Zwift when I joined the race. However, in the events page in Zwift, it did register that I was due to join the race, it just never happened. I think I will just use Zwift to join races and just use the companion to browse which race I am interested in.
