I assume you are talking about podcasts: iTunes can only read what is the the feed. If the authors of podcasts don't include episodes in the feed then they won't appear for subscribers. The Store itself can take some time to catch up with episodes since it checks the feeds periodically. You can subscribe to a podcast to compare what you see in iTunes with what you see in the Store. If you don't see an episode when subscribing then it's most likely not in the feed.
However if the recordings you are talking about are purchased series in the iTunes Store then you are asking in the wrong forum and should ask in the iTunes forum. It's also better to start your own thread than tacking onto a thread which is not directly related to your problem.