you need to add cronjob for collecting traffic data 
use crontab -e to add an 'every hour' cronjob:
10 * * * * /usr/bin/php -q /var/www/html/whmcs_dir/modules/servers/Xenica/cron/cron.php

if you have so many VMs , and this file take a long time to finish , try to:
- disable any not used servers on whmcs 
- you can separate cronjob into 2 files (every hour , and every day) like this 
10 * * * * /usr/bin/php -q /var/www/html/whmcs_dir/modules/servers/Xenica/cron/gettraffic.php
30 2 * * * /usr/bin/php -q /var/www/html/whmcs_dir/modules/servers/Xenica/cron/getUUIDs.php