In my experience, the webmasters that are hard to get a hold of are a pain in the ass to deal with once you do, which is really too bad because a lot of those guys could be monetizing their traffic infinitely better if only they'd get a clue.
I've seen the same thing on the CN where I created two different campaigns; one with managed placements and one with automatic placements with the same goal of getting ads on 'site x'. Same ads, same domains, etc., but in some instances I'll get 0 impressions via managed placements and shitloads of impressions via auto placements. It's bizarre.
I seem to remember (as a publisher) there was some setting in adsense to allow or deny direct placements, and so I had assumed that's what was going on (publishers weren't allowing placements), but now that I look through my own adsense account I can't find the setting.