Browser Caching Issue

I enabled browser caching, and it tested to work properly. However, I still get a message in Hummingbird under Browser Caching that it wasn't properly configured. I already re-checked this.

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
If re-checking and restarting does not resolve, please check with your host or open a support ticket with us.