Or of mod_deflate is not enabled, also I prefer the following for your htaccess... note that some servers will need this in the assets directory such as includes/templates/your_template/css/
Code:
ExpiresActive On
ExpiresDefault A2592000
<FilesMatch "\.(ico|gif|jpg|jpeg|png|flv|pdf|swf|mov|mp3|wmv|ppt|mpg|pdf|eot|svg|ttf|woff)$">
ExpiresDefault A29030400
Header set Cache-Control "max-age=604800, public"
</FilesMatch>
<FilesMatch "\.(xml|txt|html|js|css)$">
ExpiresDefault A2592000
Header set Cache-Control "max-age=604800, public"
</FilesMatch>
<FilesMatch "\.(php|cgi|pl|htm|html)$">
ExpiresDefault A0
Header set Cache-Control "no-store, no-cache, must-revalidate, max-age=0"
Header set Pragma "no-cache"
</FilesMatch>
<ifmodule mod_deflate.c>
AddOutputFilterByType DEFLATE text/text text/html text/plain text/xml text/css application/x-javascript application/javascript
</ifmodule>
~Melanie