Have you checked in the ACP to make sure the cron is running? If it has a past date, then it hasn't ran.
In the ACP go to Tools -> Cron Entries and look for Rebuild Forum Statistics and see if it has a future date or a past date.
If a past date go ahead and run it and then check it again the next day after the scheduled time. Then if still having issues first part of resolving is to see if you have any add-ons that deal with the statistics.