One of our WordPress-based sites was not syndicating properly to TwitterFeed, an app we use to push WordPress updates to Twitter and Facebook. Further inspection revealed the RSS feed was blank when access via our browser.
The problem: the RSS feed was not validating properly, thus showing as blank in reputable browsers like Firefox and Chrome. We used Feed Validator to troubleshoot the feed, which found an invalid character in one of the posts:
XML parsing error: <unknown>:81:1: not well-formed (invalid token)
Deleting the bad character–probably an artifact from cutting and pasting rich text from email–solved the issue. Hope this helps others with empty RSS feeds.