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there are times that wide sky appears but not the large rectangle on the center. sometimes a large rectangle will appear but not the wide sky?
am i doing wrong on my layou? have anyone experienced this?
when google says "up to 3 ads units" does it mean any combination in any places? hope you can help me. thanks
If you have a table layout this might explain why the large rectangle sometimes doesn't appear, but I have no ideas about why the wide skyscraper does not appear.
I use the 'alternate ad' feature from Google to remove the white space, so users get a slightly different view of the page in these circumstances.
Also, if you just added them to the page, it can sometimes take a day or two to display ads in second and third ad units.
In this case the wide skyscraper cell will load first and Google's script will link to Google to request ads. The large rectangle cell will load next and send another request to Google, but there may not be any ads left to display.
With a div layout you can have more control and can make the large rectangle load first, although it stays on the same position on the screen. But this wouldn't necessarily solve your problem.
One check you could do is to use the Google preview tool to see how many ads are available for your page in different countries. If there is only a few then you will never fill both ad units.