GZIP compression problem - deflate enabled

Hi there,
Hostgator have confirmed that the deflate module is enabled on my dedi VPS hosting but I get an error when I run the GZIP compression test in Hummingbird.
The error I get is as follows:

- 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

HTML appears to be disabled but Javascript and CSS are enabled.
I know very little about servers and am not very familiar with WHM so I'm not sure what to do.
Any advice would be appreciated.
Thanks,
Dave