General NewsGroup Question

I am a little confused about how NewsGroup works. Most of the time when I check on the most recent Seinfeld articles I get the option of 'Mark Thread Read' and then moving on so I can catch up. However, several times I have gone into the Newsgroup and the options are different - Post Mail, Catch up articles, Show Read articles, Subscribe and unsubscribe. I can't mark anything read and therefore can't 'catch up'. Does anyone else ever have this problem? Am I doing something wrong?