How to get Responsive Image Styles to work with existing images?
What web server are you using? If nginx you might want to look at your site config.

I have the following in my drupal nginx config:

  # Set up a rewrite.
  location @rewrite {
    rewrite ^/(.*)$ /index.php?q=$1;
  }

  location ~ ^/sites/.*/files/styles/ { # For Drupal &gt;= 7
    try_files $uri @rewrite;
  }

  location ~* \.(js|css|png|jpg|jpeg|gif|ico|svg)$ {
    try_files $uri @rewrite;
    expires max;
    log_not_found off;
  }


Since you said you never used image styles before, I assumed you might be missing these in your configuration.