Why is there no traffic shown for some of the Plesk domains in the Service Level Manager while traffic is available for all other Plesk domains?
Client's traffic values are zero in API response to customer/get/dataset/stat. It caused by internal software issue #PPPM-308
The issue was fixed in Plesk 11.5, please consider to upgrade Plesk to 11.5 version.
If upgrade is not an option, you can use a hotfix:
Unzip, backup and replace
/usr/local/psa/admin/plib/class.TrafficData.php with attached file.