The good thing about Ajax is that it enables updating of content within the page
without performing a full browser refresh but it also causes problems. The problem erupts at the development stage. A site developer using Ajax has controls over what part of web page gets refreshed and when it happens. As only parts of the page get updated via Ajax, an advertising block may or may not get refreshed every time.

Problems occur with counting page views and impressions. If a page does not reload, it becomes difficult to define what a page view is. In case a page is not reloaded it is left to the developer to refresh or Load new advertising.

In order to counter this problem one should avoid time based advertising refreshes and if that does not seem possible, one should keep documentation of how the refresh rates have been set and communicate these rates and changes that might take place with the refresh.

Thnx Eric Picard