WWWCENSOR is just a simple sample application which demonstrates how certain connections can be selected and blocked. It’s primary audience are developers, not end users. If you think to use WWWCENSOR as a base for your content filtering application then you can trace its code with the sites where it does not work and check why these sites are passed.
Anyway, I have an idea why it may not work in some cases you noticed. WWWCENSOR uses single byte ASCII encoding when searching for the pattern to block. If the web-page content is UNICODE encoded, an example, then it won’t be able to find it. As I have mentioned above this is just a simple sample.