Google Adsense has a bot/crawler just like Google Search does.
You'll need to wait for that bot to search your content again before you get different ads displaying. Adding new pages such as news items should get your new ads displaying before the old pages are crawled again.
The suggestion for using <!-- google_ad_section_start --> and <!-- google_ad_section_end --> in your themes is a good one.
You can also specify: <!-- google_ad_section_start(weight=ignore) --> with the weight of ignore in your blocks and the footer so only your original content is considered for the ads.
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):
Unix / 2.0.46 (Red Hat) / 0.9.7a / 4.1.9-standard / 4.3.2 / 188.8.131.52