On May 11 Twitter removed the ability to see @ replies from your Followers to another Follower that you do not currently follow (confused?). If I followed @someone and @someone replied to @sometwo I will no longer receive the Tweet from @someone in my Twitter stream since I myself do not follow @sometwo. There was quite an uproar about the removal of this reply setting. It was a great way to find and connect with new people as well as gain additional followers. This is only affected when the @ reply is at the beginning of the Tweet. There has been a trend of using the ".@" reply so that everyone sees your replies. It is recommended that you use the ".@" reply until Twitter remedies the issue.
Read the blog threads on the official Twitter blog about this issue.