Browser Caching not working

Hi guys,

I am having an issue with the browser caching on a site. Once I enabled it I am faced with this message;

Browser Caching is not working properly:

- Your server may not have the "expires" module enabled (mod_expires for Apache, ngx_http_headers_module for NGINX)
- Another plugin may be interfering with the configuration

I have checked my .htaccess file and it has ExpireActive On

Not sure what else it could be. I have enabled support access for you in case you want to take a look.

Thanks so much!