My site is: http://truyentranh.com. The text links was showed but I cannot validated it, please check.
I can not help but do know the code has to be on each page of that web. If Google has pages in the web indexed that no longer exist, there can be a problem. Some more knowledgeable will be along at some point in time. Good luck.
It's here: Ads not found on http://truyentranh.com/ Site failed automatic validation as of 2007-02-12 (make sure URL and site type is correct - if everything is correct, make a request in the forum for assistance). If I use the subdomain: board.truyentranh.com or phongtranh.truyentranh.com, it works well.
Try http://www.truyentranh.com/ If it goes through, you'll get more weight since it's PR 5 and not PR 1.
Is there something different about the footer (wherever you are inserting co-op ads) of your homepage from footer of all other pages? I have some sites where footer on homepage is hard coded but footer on other's are included. There has to be some subtle difference since you can get it to work on subdomains.