I've seen clicks disappearing for several years now, yet it only seems to have become an issue on this board recently. The official statement from G doesn't give any clues.
However, some wording about protecting Adwords customers that appeared some years ago - and has since been changed - was a bit more enlightening. I can't remember the exact words, but it was about click quality.
Clicks alone don't mean anything to Adwords customers unless specific goals are achieved. Adwords customers are the ones who actually pay Google and their ROI needs to be protected.
The wording a few years ago talked not about click fraud, but about low quality clicks where the clicker was unlikely to do anything else after clicking through to the site.
I have a low volume site and what I typically see is two clicks appearing in quick succession on the same page. Because of the low volume I assume it is the same person who clicked. He clicked on an ad, didn't like what he saw, returned and clicked on another ad.
When this happens, one or both clicks are taken back. He spent so little time on the first site that the Adwords goal couldn't have been successful so the click doesn't count. It's not the publisher's fault, but the publisher is punished.
When this subject came up here a few years ago someone said that all of his clicks from educational establishments were taken back. Presumably poor students don't buy when they click through to a site and thus their clicks don't count. Again, it wasn't the publisher's fault, but the publisher was the one who got punished.
I've also been following the comments here about not changing things and would like to offer the following analogy. If you have a business selling CRT TVs and complain about poor sales despite not changing anything, it is your fault. Who buys big, bulky TVs now?
But if you have a niche website for which there is demand for information and little supply, why should you continually change things? I make regular updates, but don't see the need for wholesale change.
Yes, people's Internet behaviour has changed, but apart from trying to replicate Facebook or Twitter, which we can't, what are we supposed to do? Some constructive criticism advising publishers what they should change, and how, would be better than simply criticising people who say they haven't changed anything.
After a very disappointing January, a disastrous February, and a disastrous start to March, things suddenly started returning to 'normal' (whatever that is with Adsense) on the 5th of this month. I haven't changed anything, apart from regular updates. At one stage I would have panicked and started to change things, thinking it was my fault. However, experience has shown me that whatever changes G makes behind the scenes, it affects my earning far more than anything I do.