jbayabas - 6:47 pm on Oct 21, 2012 (gmt 0)
Totally true. Google supports my theory:
Relevant ads should appear on your pages shortly after you add the code, but sometimes it can take up to 48 hours for ads to show. This is because our crawler has to visit your page and determine its content so we can serve targeted ads.
Per the OP's question: HALF of the pages are not showing ads -- it's because Google has not crawled those pages yet.