This week Twitter imposed a change to the way their system operated. In a nutshell they decided that when you Tweeted a message that started with an @reply, this message would only be seen by your followers who also followed the person the message was aimed at.
So what you might ask...
Well it now means that a well known twitter tactic of engaging in a conversation that a wider audience may find interesting is now more difficult. It means that the followers to the person with whom you are conversing will not see your @replies - in essence you are invisible to them. So no more curious twitterers getting involved in a conversation, and this is surely a step backwards.
One solution is to make sure that you never ever start a tweet with the @ symbol. Personally I am going for a . @reply.