The "Key Analysis" box should state, directly above the Quality Score, the reason the the ad not showing for the specific Keyword/Phrase in question..
Keyword Analysis: [your keyword(s) here]
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. If you're interested in additional keyword ideas, try the Keyword Tool. Learn more about building an effective keyword list.
Quality Score: Great (9/10)