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Several things may have happened. Advertisers are always dropping out or coming in. Advertisers have budgets, and sometimes they use up their budgets by noon their time and their ads go offline until the next day; so other (cheaper) ads made take their place.
Advertisers also have the ability to see the sites where their ads are place, and turn them off if they want. Some advertisers may have decided your site wasn't a good fit and blocked your site from displaying their ads.
The *only* control you have over the situation is to remove some ad blocks. Leave them off for a few days to a week, and then try again. If it happens again, then you probably won't be able to run more than one or two ad blocks on that page. Also, if you don't have link units, try one of them.
Prior ASA's on here have asked people to email and let them know where this is happening. Considering some of the reports of it being a downside to contact AdSense and have them look at a site, I have always decided this wasn't a big enough problem to wake the sleeping giant.
Then there's the old adage about someone needing to see an ad a number of times before it triggers a response. Who knows how many people click on that ad the second time they see it on your page that wouldn't have clicked if they just saw it once?
And one more thought, assume you have two rectangle AdSense displays on a page and the same ad for red widget company shows up in both rectangles during a single visit. Is that really much different than if the red widget company ad only shows up once during a visit, but shows up during every visit by that visitor? Either way, it's repetitive.
I know a lot of people were upset about perceived privacy implications of interest based ads, but I wish Google would track people and not show the same ad to someone who has already converted or stop showing the ad to someone who has seen it X times and never clicked - assuming of course there are other appropriate ads in the inventory.
We just posted to the Inside AdSense blog (the post is called, "Pageview tracking back to normal.")
You can read the details of the issue there, but basically our system was temporarily registering a pageview for each ad unit instead of for each page with ad units on it.
The issue is now resolved, and you were properly credited for earnings from clicks and impressions during that time.
The reason that in some cases the same ad may have shown multiple times on a page is that the issue at hand caused one ad unit to fail to recognize other ad units on the same page.
It is more annoying when it is image ads but it happens with text ads too.
Fixed? I somehow doubt it, cos it does not seem to be related to what ASA mentions (a very recent prob) but we will see.
[edited by: Visit_Thailand at 11:41 pm (utc) on Mar. 26, 2009]