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Using the wrong ad format could be the culprit as members of Google's AdSense content network have experienced problems in getting referral 2.0 ads to appear.

Google AdSense Referrals Hiding From Publishers

The referrals product made available by Google offers webmasters another revenue stream beyond the conventional ad placements. Referrals to AdWords, Checkout, or other specific Google (or non-Google) products pay the publisher for referring a converting visitor.

Since the rewards for referrals can be generous, the referral code has been in demand. However, a few webmasters have encountered issues with deploying the latest version on their sites.

Google's Rajiv Sud posted some reasons on the AdSense blog why this may be happening. As with some other things in life, size matters:

(M)ost 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.

A number of other reasons could affect the placement of referrals on one's AdSense units. Google only permits three referral units per page, for example. Exceeding that number would keep those lucrative referrals from appearing.

Sites that display content Google deems mature or sensitive won't spur the generation of a referral ad. Such sites would have to stick with conventional AdSense units.