Gzip/mod_deflate not Working? Check your Proxy Server
Submitted by Jeff Geerling on March 23, 2010 - 12:07pm
Recently, I was troubleshooting performance issues on a few different websites, and was stymied by the fact that YSlow repeatedly reported an F for "Compress components with gzip," even though online sites like GIDNetwork's Gzip test were reporting successful Gzipping of text components on the site.

Yslow results - not very happy.