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Anyone knows solution to this?
I think that overuse of section targeting may have something to do with it. If you use the emphasize tags to focus in on too few lines of text and/or you exclude most of the page using the downplay tags, there is too little content remaining as a basis for the link unit.
In my experience, section targeting works more or less for content ads, but the interplay of section targeting with link units is troublesome at best, maddeningly mysterious at worst.
After scaling back my use of section targeting, leaving just a couple of downplay tags on each page (to exclude the page menu, footer, sidebar, etc.), and leaving the main page content pretty much un(section)targeted, I have seen the return of link units, although they will still disappear from time to time.
Or it could just be Google tweaking some dial or changing their ad matching algorithm(s). Or on one page refresh being served ads from one ad server, then on the next page refresh being served ads from another ad server with an outdated cache (stale page info, obsolete ad matching algo, ...). Who knows?
Wide skyscrapers never fail to show on this site. It's only (horizontal) link units that often fail to show--on some pages but not others. That is, the problem of hit-or-miss link units seems to be confined to certain pages, not all. On some pages, link units have disappeared for days and weeks at a time for no apparent reason, then just as mysteriously (and with no change on my part) they will reappear again.
I think that, from page refresh to refresh, when ads appear then disappear, or a set of ad categories--call it set A--appears, then set B, then set A appears again, then no ads at all, then set C, ...
... I think it's because different ad servers are coming into play, with different sets of advertiser data, publisher data (i.e., page content, section targeting, ad types, etc.), and ad matching algorithms. One should not suppose that all Adsense ads are being served from a single ad server, or a single data center with all ad servers being perfectly and instantaneously in sync. Adsense must be a widely geographically dispersed system with significant time lags and discrepancies as ad matching algo tweaks and advertiser and publisher data synchronize among the various data centers and ad servers. How else can one explain the wild swings in ads being served from one moment to the next?
incrediBILL - I am aware of that appear/vanish behaviour and I'm trying reloading those pages twenty times a day but without any luck.
At the same time, the pages that do show adlink and other adsense ads work well, so I guess there is some particular reason why those pages work and other don't...
It really pisses me off having that nasty, empty space on those pages including index...
If you change a page, you have to wait for:
--the next visit by the Google adbot
--Google to analyze the page, then post the results to the "master" ad server/center
--the master ad server/center to synchronize with the rest of the ad servers/centers
This process can take days and even weeks.
If you make a change (add section targeting, tweak page contents, etc.), don't be discouraged if you fail to see the expected results immediately or soon. Maybe the ad bot hasn't visited the page yet, or you might just be seeing outdated results from a laggard ad server. You must be patient.
Having said that, I must add that even after careful study over several months, I have important clues about how section targeting works, particularly with respect to link units, but no real understanding. Using section targeting, sometimes I get good results, sometimes bad, often with little rhyme or reason. It's all a black box in a locked room in an underground vault below a high-security fortress. It's all so (needlessly? by design?) mysterious and impenetrable!