MikeNoLastName - 10:08 pm on Feb 2, 2012 (gmt 0)
The preferences and geotargeting determining ads REALLY sucks for destination travel sites. Someone from metrocity comes into the website looking for information about podunk and most of the ads come up about metrocity rather than podunk.
Oh, and I've been using the start and end tags for years and use at least one for each ad per page. I've also been known to target as little as a single short sentence with keywords and never had a problem. I agree with the other poster about probably not enough inventory for your specific ads, or GAdsense is in test mode again, testing to see if alternate ads work better for your site.