Why adsense sometimes don't show ads where is a lot of text? Or for example u come to site and u can see ads, u come back after 5 mins and u can't see any ads on main page, again u come back after like 5 mins and u can see ads again. It's not all the time, but it happens sometimes.
Try AdSense Sandbox utility to check if your site is not banned. Search google about it. I had the same issue with one of my sites (503 error) and then I realised, that the site was banned.
to many ads? maybe script bugs! i've used joomla for one of my websites and when the site was displaying with "http://www" some ads were not where thay supposed to be and when the website was displaying without www the ads were ok!