Twitch recently changed their api and we had some updates to our streaming plugins. Are you using the "Justin.tv/Twitch" plugin or the plugin just called "Twitch" ?
On v1.1-1.2 you can see that if you go to your channel list (where you added the channel) click on add and on "more ...". The Plugin Browser will open and you can check which plugin is installed or can be installed.
In our new Beta 1.3 you can find it under Broadcast -> Add channel ... -> More...
I can also recommend trying the Beta: http://www.xsplit.com/blog.php?post_id=365