GZIP not working properly

Hey guys,
I can't seem to turn on GZIP compression on my site with the Hummingbird plugin. Any ideas?

gzip is not working properly:
- Your server may not have the "deflate" module enabled (mod_deflate for Apache, ngx_http_gzip_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.