TweetCaster respects the privacy of its users. Those who have set their accounts to private expect that their content will not be shared publicly. Although TweetCaster previously allowed followers to retweet this content, our friends at Twitter have asked us to stop. And we agree that it is the right thing to do. :)
So that capability has been turned off. TweetCaster is not the only one making this change–it is our understanding that other Twitter apps are turning off this functionality as well.