On my website I have ads appearing on only some pages. I placed the code in the main index which is used on EVERY page. myroommatesucks.org - it makes space for the banner ad, but nothing appears myroommatesucks.org/best - same issue ^ myroommatesucks.org/worst - appears myroommatesucks.org/random - sometimes appears myroommatesucks.org/about - appears myroommatesucks.org/contact-us - appears myroommatesucks.org/faq - appears Any idea why it would only be on some pages? Do I have to wait even longer for ads to propagate for those pages as well? I haven't heard of this happening to anyone before and I myself have never seen it before.
OR could be because of stop words you have on your pages sometimes. Words that make adsense no display any ad until its bot visited your page and determined if it has bad content or if the stop word was only part of an PG appropriate content. Also could be because until the bot indexed your pages, adsense can't determine the main topic of your page and so delay displaying any advertizer.
some times google cant findout which ads will be perfect for u r keywords so they place google service blank ads ...
If you want passably related ads before the bot comes around, the adsense script does look at meta keyword and description tags too. And for people that say meta tags don't count, I had a site where one of the keyword was 'pregnancy' but the domain had 'knowledge' in it, when I put adsense on that site all the ads were for knowledge base tools, data warehouses, scrapers etc, I put 'pregnancy' in the meta keywords and boom, instant change to baby ads.