There was a bug in versions of Plesk prior to 8.3 where the AWStats statistics for the previous months were unavailable. It was a bug within Plesk’s AWStat’s implementation, and it was fixed in Plesk 8.3.
However, the fix only corrected the issue moving forward after the upgrade. There was no automated way to rebuild the previous months’ statistics, even though the AWStats data was right there on the disk!
I saw this blog post about the issue, and the fix is quite elegant: