This was annoying. I'd noticed some feeds that I read weren't showing up as updated in NewsGator Online. I checked the "My Feeds" page in my account settings, and I found a fairly large number of them marked as "error at source" or "not authenticating", with the system indicating that it hadn't checked them since mid-to-late May. Fortunately, there's a "ping" button that lets you make the system go and check the feed again. In some cases, I found that the feed location had changed without a redirect being provided. In other cases, the system must have hit a server error and decided that the feed was stale. I ended up deleting a few feeds, fixing the address of another ten feeds, and just refreshing the rest. I hope the system's unclogged now.
If you use NewsGator or FeedDemon, you might want to check things too just to see if it's all OK.