As you may or may not have noticed your Twitter feed would have stopped working as of this week. This is due to Twitter updating their API and not allowing 3rd party apps to pull in your latest tweets. There is a workaround with using a PHP file but as Tumblr is only HTML this is not possible.
I am currently updating all of my themes that feature a 3rd party Twitter feed with Tumblr's own feed.
Keep an eye on my Twitter feed for updates to the theme your using. https://twitter.com/ChristineWilde
If you have any further questions please do not hesitate to get in contact via the appropriate support channel.
Themes that have been updated so far:
Optic - http://themeforest.net/item/optic-responsive-tumblr-theme/2784293