For SCSS to work you have to compile it after each save.
I recommend installing Ruby on your computer and using the command prompt to "watch" the file for changes.
basically something like this.
sass --watch styles.scss:styles.css
This will tell SASS to watch for changes on any files associated with styles.scss or whatever you named your master SCSS sheet. changes to _settings or _mixins etc will all be picked up and re-compiled after the save.
Check out this link for some basic info: http://sass-lang.com/guide