If you are having trouble with a specific feed not loading, click edit on the feed folder and remove that specific feed and click Save. It's important to remove just one at a time and click Save after each change. Wait for a few minutes and see if the other items start to load. From time to time a hiccup from one feed will cause a delay in the other items loading. Typically, it's the first URL that isn't loading that is the culprit.
If that doesn't work, you may want to remove all the non-loading feeds, one at a time, and again, click Save after each one.
Then re-add them one by one and click Save after each one. This can help you figure out which feed is causing the problems, and you may not want to use that feed as a source.