Browser caching not working

Hey guys,

I'm trying to give the site a bit of a performance boost using HummingBird. For some reason, when I try to enable browser caching, I'm getting an error which says:

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 enabled caching manually via the .htaccess file using the settings given to me by the plugin.

I haven't restarted the service, not sure it's necessary?