Different window sizes is only the one side of Responsive Design. There are another things that may cause the problems. All modern browsers on desktop work ok, they have support for almost all the most important and commonly used css features, but on mobile devices it doesn't look so good. Mobile browsers doen't support all features that are available for desktop browsers. I created a webiste with RWD and on dekstop it works very good (I tested it by resizing browser window) but I don't know if it works on mobile devices (due to css features not supported by all mobile browsers). I put a request in 'Website Reviews' for my website but i didn't received any feedback yet so I still don't know if it works.