Well, all barring one group of keywords, which in three months have had over 100,000 impressions.
Could Brand X have something to do with this? Other advertisers are appearing for these terms just fine, and the Traffic Estimator does predict traffic (admittedly a suspiciously low traffic, considering how with high volume terms it has stupidly high estimates. I put in "Brand X" with CPC at 8p and it predicted 900 clicks a day!). Anyone else had anything like this happen? Can Google stop a keyword from showing without actually disabling it?
Standard Disclaimer: It is really hard to answer this sort of question without seeing the account, so your best answer is likely to come from AdWords support, who can do so.
With that out of the way, here's my theory: most often in the past when I have been asked this question, it turned out that the same keywords are used in another Ad Group somewhere else in the account. Since only one ad will show for a given keyword search, the AdWords system picks one usage of the keyword, and if nothing changes, that usage will be the one that gets the clicks and impressions. The other(s) will stay at zero.
(BTW, you can search your account for dupes by using the 'Search my campaigns' tool at the top right of Campaign Management pages.)
Also, here is some background info on which usage of a keyword the system will pick, in cases in which a keyword has been used in more than one Ad Group:
* The system will first default to the usage of the keyword in which it has the highest Max CPC. (And this is my guess as to what is happening in your case.)
* In cases in which all usages of a keyword have the same Max CPC, then the system defaults to the case in which it has the highest CTR.
If this doesn't turn out to be what's happening in your account, well, then maybe it's time to contact AdWords support. ;)
AWR
It is possible if you added this keyword while it was an 'unallowed' term, that it was disallowed, but isn't marked properly in your account. In this instance, you can just delete and readd the keywords (or you might need to duplicate the group), and it should run fine.
If that doesn't work - write AdWords support.
<added>AWA beat me to the answer, started writing, submitted, and *poof* there's this other post there :)</added>
Overture will match singular searches, plural searches, possessives, hyphenated phrases, misspellings, semantic matches or near-matches, and more. It will show the same set of results for many variant searches, and it will lump them together in the reports, too.
Thus, if there were some searches for "widgets blue", some for "widjets blue" and some for "widget-bloo" Overture would show "blue widgets" results, and they would show up in the reports and the suggestion tool as searches for "blue widget". You could end up optimizing for totally useless target terms if you depend on Overture's tool for anything but setting up your Overture account. I'd be willing to bet big money that's where your puzzle originates.
Anyway, I resumed the campign and tried the ad diagnostic tool.
Ad not shown ¦ Reason: Our system is experiencing technical difficulties.
So that's cleared that up then ;)
Doesn't matter, it's not important (the keywords were in area that wasn't doing well, so it's no big loss). Just odd.
Thanks for your help people.