RSS feed not updating

I have a phpBB forum which outputs atom RSS feeds. They update fine and I’ve never had a problem with them. I used the web address for the phpBB rss feeds and pasted them into the RSS widget in wordpress. It displays the feed just fine when I first implemented it, but when there is a new post/thread on the forum, the phpBB RSS feed updates itself, but those updates don’t display in the wordpress widget. How can I get the RSS wordpress widget to display the updated feed when it changes?