If your session directory (most like /tmp as you said) is not writable, then it won't be able to save and will have to regenerate a new one each time. Here is how you can verify it:
if (!is_writable(session_save_path()))
{
echo 'Session save path "'.session_save_path().'" is not writable!';
}
If that is the case, you will need to have the server admins give what ever user your webserver runs as permission to write to the /tmp directory.
