If awstats or webalizer is not containing any data and the client web logs are empty, the cause might be missing mounts of the log directories in /etc/fstab.
Search /etc/fstab for the client domain name. If no log mount is found, add a line for the client web site manually in this form:
/var/log/ispconfig/httpd/DOMAIN.COM /var/www/clients/clientNNN/webNNN/log none bind,nobootwait 0 0
After that do:
to verify it is mounted. After nightly cron, check that awstats or webalizer contains data.