Bloggers who have Google AdSense units set to display in their blogs often complained about missing AdSense units. I will use this blog to explain.
I have set the following AdSense units to display in this blog with the number of AdSense units in bracket):
1. wide skyscraper unit in the left column (1 unit)
2. leaderboard unit in the footer (1 unit)
3. large rectangle in between posts (6 in the main page but only 1 in the individual post page)
Note: There should be 6 ad units in the main page or homepage) because I have set the number of posts to display in the main page to be 7 (the default).
Now in you count the total number of ad units, you will get a total of 8 ad units (1+1+6) in the main page but only 3 in the individual post page (permalink).
Google AdSense only allows a total of 3 AdSense ad units in any page and so in the main page, the extra 5 ad units get suppressed and you only see 1 wide skyscraper unit in the left column, 1 between the first and second post and 1 between the second and third post giving a total of three.
However, in the individual post page (permalink), there is only 1 ad unit immediately after the only post, 1 in the left sidebar and 1 in the footer giving a total of 3 (allowed) units with none suppressed.
Now if you have set AdSense units to display between posts and you don't want any ad units to be suppressed and you have similar settings for the AdSense units, all you need to do is to set the number of posts to display in the main page to 1 and only 1.