Untill people visit the board they have no value in the enhanced last seen time so it's overlooked untill the field is not null.
The new user field for the last seen time will be updated with every call to send_header so if you have the additional code in place it should be working.
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.