I was recently working on a development version of a Drupal site, and I couldn’t get respond.js* to work for the life of me. It was working fine on our live site, and I couldn’t figure out what I had changed on the development site that would break respond.js.
But respond.js works by requesting a fresh copy of the CSS via ajax, and when you have fifteen different CSS files from plugins and modules it stops working correctly. Compile all those and it should start working again.
*If you’re reading this, you probably know that respond.js is a polyfill that lets media queries work in browsers that don’t natively support them. Typically it’s used so that your site doesn’t look like a total mess in IE 7 & 8.