AdSense Preview and Actual Ads

[cubic]

We are running an information technology web site with daily updated news items and articles. In the last three months we were testing Adsense to find how to fit the ads within the content. During the test, we used Google AdSense preview tool and discovered few things:

1.Our site almost never shows ads from the Google AdSense tools.
2.Some ads are only displayed on skyscrapers and not in 300x250 boxes.

Does somebody can clarify it for us? We have been using AdSense since 2003 and it’s the first time we have such situation.

[ericgtr]

I have found the Adsense preview tool to be off the mark of the displayed ads most of the time myself. I am not sure why this is so I rarely use it anymore.

As for #2, It's most likely because Adsense doesn't have enough ads to always populate your 300x250 box and when it doesn't, it leaves it blank. Personally I would rather see a public service ad or anything other than a blank space.