Why am I not seeing my ads?
The answer to these questions and several other variations is called "Personalization".
This is a new concept that was rolled out from Google in December of 2009. In simple terms, Google is trying to "personalize" YOUR experience. That is to say that they are trying to show YOU things that YOU seem to be interested in (based on their observations of your computer usage). If you have done this search several times and have never clicked on the ad, then Google says "hmmm... he/she must not be interested in that." As a result, they might not show you those ads.
This is a great information page from Google about Personalization. It includes a video that explains the concept further:
This article from Business Week’s blog further explains the concept of "personalized search":
This paragraph from this article (near the bottom) explains this concept well.
Recently, Google quietly rolled out a feature which may have even greater impact on Web users — though many are unlikely to notice. With something the company calls Personalized Search, Google will start showing different search results for different users, depending on which links they have clicked the most in the past. In theory this means that eventually, a car lover and a zoologist typing "jaguar” into the search field will wind up with two different sets of search results.