It is very likely because you are using the old Twitter API, for example:
<script type="text/javascript" src="http://twitter.com/statuses/user_timeline/USERNAME.json?callback=twitterCallback2&count=4">
You will need to update your code to the new API:
http://api.twitter.com/1/statuses/user_timeline.json?screen_name=USERNAME&include_rts=true&count=4&callback=twitterCallback2
This requires that you edit the code of the theme. Many themes suffered this issue for a few months ago when Twitter made their changes.