The code looks ok, everything is set up correctly - but still, these users are getting blank ad units. That costs people money and creates a poor user experience. Some folks in this WebmasterWorld thread say that the problem only happens in Firefox, others say Chrome.
"Even when refreshing the page, the ad spots remain blank," writes member FourDegreez. "If this is happening to a lot of users, it would explain why I'm seeing a big drop in earnings in recent days."
WebmasterWorld member WMaster99 adds, "I'm also seeing blank ads from time to time. Under Clicks > Impressions, I'm seeing that the impressions don't match the visitors since the blank ads started appearing."
Anyone else out there seeing blank ads? Heard anything from Google?