If the local mirror server can reach the update server and download the master.idx, check proxy and perimeter firewall logs to determine if the issue is with downloading larger files
Ensure there is no packet inspection of traffic between the local mirror and CB's update server performed by firewall or proxy
If issues persist, reboot the local mirror to ensure there are no process holds on update files preventing the update
If still not resolved, create a new local mirror (requires a new download directory and reconfiguring web server to this new directory)
The new local mirror is created when running the command in Step 4 (both Windows and Linux) from the Local Mirror KB
Additional Information
Typically the issue is caused by the scheduled update of the local mirror server not running as expected
ProcMon and pcaps can also be used to investigate the local mirror update process