GRMetrix reports that browser caching is enabled at all my sites that use WP Super Cache. Your issue probably is due to web server configuration.
The first troubleshooting step is to check the GTmetrix report to see which types of files are not leveraging browser caching. If they are related to ads, that will be by design and the only fix will be to remove the advertising. If instead they are files being served by your web server, then provide your web hosting tech support staff with a list of the file types.