Google allows up to three AdSense for content units on the same page, but it might very well happen that you can only see one – or none at all? Why? And what's the solution?
When Google serves ads, they do it by looking at your title, your filename, e.g. www.mydomain.com/no_adsense_ads_here.html, your content etc. Lets say that you have written about green tea leaves. Google has 7 ads with these keywords, and you've put up three units:
* First unit is a large rectangle, showing 4 ads.
* Second unit is also a large rectangle, showing 4 ads.
* Third unit is a large tower, showing 5 ads.
That's a potential of 13 ads, but Google only had 7. So probably you'll have the first unit filled with 4 ads, and the second with 3 ads. On the third Google shows a Public Service ad. You do not get payed for Public Service ads.
Make your own ad units
The solution to that problem is to create your own ads to use, when Google has none. Just make one or more HTML-pages with small ads for things that has something to do with your homepage. You can find products at ClickBank or Amazon and use their affiliate programs to make you money. When you are creating the code for the AdSense unit, fill in the link to this alternative ad. Whenever Google has no relevant ads for your page, they will serve your own ads in stead.
Google always serves you ads from the top of the page and down. This is good. Because most clicks will be made above the fold, i.e. on the part of the page that can be seen in the browser without scrolling down.
No picture ads first
But it could cause you problems, as Gal of 60in3.com has pointed out in this blog posting: http://blog.outer-court.com/archive/2007-04-17-n54.html
Gal made the first ad unit a picture ad only, but the problem was that if Google had no picture ads fitting the content of the page, they didn't serve any ads at all, although the following units should be text ads.
So avoid using picture ads as the first ads on your page. Perhaps you should avoid picture ads at all? Joel Comm thinks so. I think so. From my experience they rarely get any click, so why use up space to host them?