Rajiv Sud from the AdSense Publisher support has a blog post explaining why referral 2.0 ads are reluctant to appear on publishers’ pages. Here are summaries about what might be happening:

1. Not referral ads are available in all sizes. Select categories or products for your referrals before selecting a size.
2. The particular referral ad you’ve chosen is not available in your country.
3. You’ve selected the “Pick best performing ads” checkbox for referral where the advertiser has run out of budget or ended the campaign.
4. You have more than three referral units on one page.
5. Potentially sensitive or mature content on the page.
6. Modified referral ad code.

The next time you stumble upon this problem, make sure you go through the simple “checklist” to make sure that one of those common reasons is not part of your problem. Of course, you can shoot an email to the support team for further support question.