Gzip compression isn't working

Gzip compression is not working on my site. Hummingbird says the following:
Gzip is not working properly:

- Your server may not have the "deflate" module enabled (mod_deflate for Apache, ngx_http_gzip_module for NGINX).
- Contact your host. If deflate is enabled, ask why all .htaccess or nginx.conf compression rules are not being applied.
If re-checking and restarting does not resolve, please check with your host or open a support ticket with us.

mod_deflate is enabled on the server and all the other sites in the server have Gzip enabled and working. The htaccess rules are in the htaccess file.