I have been on the phone daily with Google trying to get an answer or at least a general idea of why some of our keywords (the important ones) are not showing ads. I get this message:
Ad Showing:
No
Your ad shares a similar Display URL with a competing, higher ranking ad.

Now after days of chatting with Google this is what they said is most likely the issue..(below)Sorry for the long message but it sounds like a site redesign...any have this issue before and what did you do to fix?
******
The message you are seeing in the Ads Diagnostic Tool
is correct which reads, "Your ad shares a similar Display URL with a
competing, higher ranking ad." I'll explain what this message means and
how you can make changes that can help your ad show.

Google believes that search results pages with multiple ads leading to the
same site provide less relevant results and a lower quality experience for
our users. Accordingly, if two or more domains lead to the same page, our
system can still ensure only one ad leading to this page will show. The ad
that shows will be chosen based on its Ad Rank for relevant search
queries.

To have the status of your site reconsidered, please refine your page so
that it contains a substantial amount of unique content, as well as a
unique overall look and feel. We'd be happy to review any proposed changes
to your web page, and we appreciate your efforts to comply with our
policies.

If you leave your site as is, I recommend working to improve your Ad Rank
in order to better compete with other sites using a similar Display URL.
Your ad's rank on Google and the search network varies per keyword and is
determined by your matched keyword's cost-per-click (CPC) bid and Quality
Score on Google in relation to other advertisers' CPC bids and Quality
Scores.

To improve the rank of your ad, you can increase your CPC bid and/or
refine your ad text and keywords to help increase your Quality Score.

You aren't allowed to run 2 ads for the same URL. If you have 2 campaigns running bidding on the same term, the most effective of the two ads will run.

Even if you are running 2 separate accounts and bidding on the same URL it will only show the most effective of your ads from either account.