The AdSense Blog has made a post explaining "publishers having trouble viewing referral 2.0 ads on their pages". Here is what they say:
First, keep in mind that not all referral ads are available in all sizes. For instance, most horizontal referral ad units smaller than 180x60, square referral ad units smaller than 125x125, and text links are only available at this time for Google products such as AdSense or AdWords. This means that if you generate code for referral ads in an unsupported size, you won't see any referrals shown on your webpages. To avoid this issue, we recommend first selecting categories or products for your referrals before selecting a size at this time.
Similarly, you may not see the referral ads you've selected on your pages if you've grouped a number of referral ads into your Ad Shopping Cart which are each available in different sizes. If this is the case, you may wish to try regenerating your referral code with a particular ad format in mind -- this way, you can be sure to select products available in that format.
Here are several other reasons your chosen referral ads may not show:
- The particular referral ad you've chosen is not available in your country. When generating your code, you can view the ads available for specific countries by clicking the 'change' link above the referrals wizard.
- You've unselected the 'Pick best performing ads' checkbox for a referral where the advertiser has run out of budget or ended the campaign. To take advantage of the available inventory of related products, we recommend leaving this box checked.
- You've added more than three referral units to your page. Our current policy allows a maximum of three referral ad units on any policy-compliant page.
- Our system has determined that your pages contain potentially mature or sensitive content. However, as your content changes, you may begin to see referral ads appearing.
- The referral ad code may have been modified. Be sure to copy the code exactly as it appears in your account and paste it directly onto your pages.
Google, AdSense, Referrals, AdSense Features, Troubleshooting