Try using the header set
for caching by setting the expire date in the future. You can use filematch for specific file types. A year typically a good time. Edit the file types as necessary.
<filesMatch "\.(ico|pdf|flv|jpg|jpeg|png|gif|swf|js|css)$">
Header set Expires "Mon, 15 Apr 2015 20:00:01 GMT"
</filesMatch>
This definitely works for me.