1. How long have you been having the issues with ads not serving on the particular pages having problems?
2. Is it possible that the topic of these pages may be being determined by YPN to be a keyword that doesn't have any advertisers? (i.e. related to your core keyword with advertisers but slightly different enough to be getting associated with another keyword?)
3. How many ad units are showing on the pages?
4. Have you tried changing where the ads are located to see if that makes the ads show up? (i.e. moving them from a sidebar to within the content, etc)
5. Have you tried changing some of the on page copy (title tags, content surrounding the ads, heading tags, etc) to see if that helps at all?