If there really are no "stop"/"bad"/... words in there - which is not easy to determine, there are a few other things that can block ads:
- robots.txt blocking google's adsense mediabot, or other thing that disallows google to get the content on the page
If it were just one URL that had the problem (so kind of the reverse of what you have), I've dealt with that in the past with a domain that had been parked before I got hold of it. It refused to show ads on some URLs, and copying the page to any other URL would show the ads (permanently), just not the one logical name. After contacting Google support they told me they had removed the domain's prior blocking it had from when it was of one the previous owners.
Google has an online troubleshooter for those of us that have trouble with ads not displaying.
Suggest you give it a spin.