I've fielded several support issues with your same symptoms. For them, it turned out to be a backlog of headers that have accumulated due to the spam attacks on the newsgroups.
You can tell by checking your cache display at the bottom of the Newsbin window in the Status Bar. It'll look something like "Cache 90/100 (xx)". If there are numbers in the parenthesis, it represents a backlog of headers waiting to be processed.
If this is the case, you should stop any header downloads and let Newsbin sit until the backlog is processed, then you'll see the new posts. A better solution would be to blow away the backlog and implement the filter that Quade pointed you to. The procedure to set the filter is also in our
help wiki.To remove the backlog of headers, go to the Newsbin Data Folder and delete all the .gz files in the Import folder. You can find the Newsbin Data Folder by going under Options -> Settings within Newsbin and then click the "Open Data Folder" button.