Magento 1.9 – Cache is On, but var/cache is Empty?

Move your sessions in the memcached. It's saving sessions in memory, so it's working faster than from filesystem. Also you will have ability use few servers instead one. Memcached has one issue. When server reboots - users will lose their sessions. More information here: http://magebase.com/magento-tutorials/magento-session-storage-which-to-choose-and-why/

As you're not using a file based caching mechanism, Magento has no need to store files within the /var/cache/ folder - these are stored in Memcached and Database given you're configuration. If the solution previously gave you output in the /var/cache/ folder, then it's likely the configuration was cached from the previous set up.