Webconsole user diagnostics: "Current Mailbox Size" missing

Hi Forum,

why does the administration console show the following line for most users? For a few users this entry is missing...

Attribute ID    Description                 Value
61238 Current Mailbox Size (MB) 187

Is this a bug or are these accounts some kind of defect or is this field just hidden or....?
We definitely need this statistic for ALL users :)

Thanks for any suggestions,