I bought 3 standing tickets over two transactions. Just like you, 2 arrived but the single ticket never did. I've just checked 'My Ticketmaster' and it says that the order is cancelled!!! They never emailed me to tell me that it was cancelled, the only email they ever sent was to confirm that the booking was successful :cry:
I've only just trawled through my credit card bills and seen that they also never took any money for that single ticket so I doubt that they'll honour it.
I don't know wthat to do now, I had no idea about that rule - and I need an extra ticket :cry: