I was trying to send a text message consisting of an abbreviation I had set up, but I would type the two characters and the abbreviation was not appearing. Is there a cause? In the past, I have restarted my phone and it has worked afterwards, but I'd rather not have to do that if there's another way.
This is a bug.
My personal hypothesis is that the root cause is the new iOS 6 iCloud syncing for text abbreviation sometimes fails, thus breaking the whole abbreviation system.
Since it appears to be quite common, a fix is hopefully coming soon. To increase the chances, be sure to report the bug here: http://www.apple.com/feedback/iphone.html