I use Mac Mail as my default RSS feed reader. Recently, the feeds stopped automatically updating even though I set the interval to be every 30 minutes in the preferences. I was still able to manually update each feed by right-clicking and choosing “Update”, but this soon became tiresome.

After a lot of searching on the internet, I finally found a solution that fixed the problem, which works.

Delete the file ~/Library/PubSub/Clients.plist and then restart Mail. Your RSS feeds should now auto-update again, and I think also when you click on the “Get Mail” button.

Source: http://discussions.apple.com/thread.jspa?messageID=7840684