We had an odd issue setting up Mod Deflate on one of our servers this week. It turns out in order to activate compression on our php/html pages we needed to add a line in our .htaccess file to explicitly prevent gif, jpg, and png images from being compressed). I’m still not 100% sure why this worked, but the code that got all of our pages to compress is copied below:
SetOutputFilter DEFLATE
SetEnvIfNoCase Request_URI "\.(?:gif|jpe?g|png)$" no-gzip