When you add a podcast subscription in iTunes, iTunes will only download the latest episode. Then from there-on iTunes will download new episodes as they come available. If after 3 consecutive episodes, if you never play or sync the media to an iPod/iPad/iPhone, iTunes will stop downloading. The subscription remains in iTunes and as soon as the user clicks 'get' on the latest episode, iTunes will start downloading latest episodes again as they are released until 3 consecutive untouched episodes occur. This is all iTunes behavior, we have no control over this.