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:

Not referral ads are available in all sizes. Select categories or products for your referrals before selecting a size.
The particular referral ad you’ve chosen is […]

As an AdSense publisher, it is wise to constantly test your ad units to see which one performs best. Optimization can be done through testing, which in turn will get the most revenue out of your existing content and traffic.
At the risk of sounding like a broken record, I am going to say it again. […]

Click-through rate (CTR) is an interesting topic to bring up on discussion boards, blogs and on industrial events where web publishers gather together to talk about website publishing revenue.
As you might have read, CTR depends upon too many factors it may be acceptable for one site, great for others and lousy for other half.
There […]

Often we read in forums, blogs or articles about stunning click-through rates claimed by some AdSense publishers by using some techniques or others.
Assuming the claims are true, we rush to adapt this method hoping to achieve more or less as high CTR as the original poster claimed. Much to our dismay, often the techniques result […]

Many web publishers focus too heavily on CTR because it is simply lucrative. If we could just increase our earnings from the existing traffic and visitors, why shouldn’t we do it?
Moreover, getting more traffic after optimization usually means even more revenue. Better still, it squeezes the most out of available resources.
There are many factors […]