A fundamental flaw in Google's AdWords' platform is proving to be extremely frustrating. When purchased keywords have a low search volume, ads often do not appear in queries, even if users are searching on the exact phrase you bid on.
In this case, Google serves up a message that the quality score is great, but doesn't trigger the ads when you search on targeted keywords. Getting the ads to show requires you to bid on slightly less relevant keywords. This post provides tips on handling this glitch.