There has been a lot of discussion recently about the problems publishers are having with Google's referral ads.
Due to those discussions, AdSenseAdvisor started a thread over at WebmasterWorld providing some feedback and solutions to some of the problems.
While some problems are technical bugs, there are others that can be resolved by understanding how Referrals works.
When you pick a specific referral ad, and not a generic category, you are setting yourself up to displaying an empty ad spot. Why? Because if the advertiser runs out of money in his or her budget, Google won't show the ad. What can you do? Google suggests you "check the 'Pick best performing ads' box when generating your referral code." This will rotate different ads from that category, and if one ad is not running, it will replace it with a new ad.
There is a bug with some of these selections, AdSenseAdvisor explains:
However, if you select an ad unit size that is only available for Google product referrals and check 'Pick best performing ads,' there's currently a system error that will cause no ad to be displayed. Our engineers are working to fix this as quickly as possible, but in the meantime, if you'd like to refer users to these Google products, please uncheck the 'best performing ads' box when generating code for that referral ad unit.
AdSenseAdvisor stresses that Google is working hard to work through those kinks in the system.
Forum discussion at WebmasterWorld.