Are you missing any users or are they by user numbers which are not in the Users table. If things are missing then the table joins will fail and they will not be displayed.
One missing user (from database corruption etc...) can make threads disappear until the user number is replaced.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.