Some web site implement security policies which prevent external scripts like Readlang's from running. Google and Facebook do this, and it's impossible to load the Web Reader there. It used to be that the vast majority of news and blog sites would work, but some (e.g. BBC mundo) are now preventing external scripts from running.
The other problem that sometimes happens is that content within an iframe isn't translatable (an iframe is a web-page within a web-page, commonly used for adverts or comments).