The WordPress Plugin Tweet Old Post which is invaluable for helping you vitalize your older WordPress articles appears to have a bug in version 3.1.1 of the plugin where it will stop tweeting any posts if you have any category exclusions selected for your blog. I had category exclusions and noticed no posts were being sent to Twitter, after clearing all my excluded categories and saving my options the plugin started working again.
Meanwhile in the developers page for the plugin http://www.ajaymatharu.com/tweet-your-old-post-with-tweet-old-post/ there are others reporting the same issue and I have been able to reproduce it on 6 of my blogs. It must have happened with the most recent plugin update or maybe the one prior because I know the plugin used to work with my category exclusions at one point.
If you use the WordPress Tweet Old Post plugin, you will want to check and make sure you aren’t excluding categories or the plugin may not actually be tweeting anything on your blog.