At Search Engine Watch, member abbotsys notices that the Quality Score for low volume keywords is good -- but if you look closely at the keyword analysis, the ad isn't even running!
According to Google, those ads are not showing because nobody is searching for them:
Ad Showing - No The keyword phrase you've entered has a low search volume and isn't showing any of your ads. If more users start searching for your keyword, your ad will begin to show. You don't need to do anything
Should Google be doing this? Forum member Gooner151078 says that "In my mind, you as an advertiser have the right to have your advert showing against any term, regardless of the volume." (But one can argue, then, that an ad with a low quality score should still be showing up!)
However, others understand (and have encountered) this issue. One says that matching terms for that keyword should be added (surrounded by quotes or brackets) and that you should keep in mind to utilize long tail keywords to create more opportunities for your advertisements to show up.
Forum discussion continues at Search Engine Watch Forums.