If you are using CCM Based Monitoring there are several items to check in Call Manager
Span to PC Port
Enabled PC port
Check the monitoring Calling Search Space on the Line.
On the desktops that you are unable to monitor, go to C:\Program Files\Cisco\Desktop\bin and run the postinstall.exe. After selecting the CAD server to configure, go to the " Voip Monitor Service". Make sure that the correct IP address is showing up for the nic adapter. I had an issue after a CAD upgrade where CAD was binding to the wrong IP address thus causing an error when you try to silent monitor.
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