I have the large square 336x280 on my site, sometimes it displays 4 adds and sometimes it displays only 1. I noticed one day it displayed just the 1 add for the majority of the day. Why does it do that?
I googled it, and someone said sometimes it has relationship with ads holders, they set up the max cost per day, if other advertisers got enough clicks and reach the max cost, the ads will not shown.
I think that this has to do with the relevant ads that Google has available for the content on your site.