Why I only get public services ads? Help!!

I have a PR3 blog and already 9 months old. I have put adsense code for a while and doesn't have any problem. Until recently the ads won't showed up and only showing public service ads. I already tried to optimize my keyword. What's the problem? Can't anybody help?

 

Some times they'll banned your account

 

This happen also if you are not verify your pin

 

Nope, I don't think so, because I have other blogs in the same adsense account, and the ads is up and running.
Besides, this problem happens occasionaly not all the time, but it really bother me because sometime it happens when my traffic is flowing.

 

Then it is content related issue, not only add keyword, but add more content and keyword

 

Are there "stop words" on this particular blog? Here are some of them:
http://www.vaughns-1-pagers.com/internet/adsense-stop-words.htm

(Google doesn't have an official list - it was assembled by trial and error, I believe)

 

Well, this is new information for me.
Can you tell from my blog here, did I used to many stop words?

 

You can read through your blog faster than I can. If you look at the link I posted, you can see that the stop words have to do with violence, death, medical conditions, sex or adult content, drugs, etc.

And stop words are just one possibility. Google has other possible reasons for Public Service ads here:
https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=10035

Some other possible issues: your site doesn't have enough text content (videos and photos don't count), your site doesn't comply with the program policies, you modified the ad code, etc.