Like a lot of people on here my earnings took a dive when google introduced their brand safety policy in december. i'm getting blank ad spaces all over the place, even after the page has been visited multiple times.
i've had people visit the page in other cities, seen the ads appear, then had them disappear the very next day, maybe reappear again an hour later, then disappear, reappear... there really is no rhyme or reason to it.
the way i understand it is this: the pages are supposed to stay in adsense's index for two weeks after they've been crawled by the adsense bot. after two weeks they drop out of the index and will have to be visited by another user, get recrawled again, and then they'll get another two weeks grace.
here's what i'm thinking... if you change some of the content on the page will google automatically boot it out of their two-week index? in most cases the only thing i'm changing is the meta expiry date in the <head>. everything in the <body> is identical. but seeing as this date changes every single day, i'm wondering whether that tiny change is enough to boot it out of their index and trigger the need for a re-crawl.