There is a utility in the newer releases of UCCX that is included with CAD on the agent PCs that can help narrow down the cause of this problem.  On the agent PC, navigate to C:\Program Files\cisco\desktop\bin and you will see NICQ.exe.  Run this utility while the agent is on a call.  This utility captures packets on the NIC and verifies that the PC is receiving packets from the phone correctly.

If this tool reports that it cannot find an RTP stream, it's best to focus on the agent PC and their phone configuration.  On some phones, you may need to disable the advertise G722 codec, because this is not supported with CAD monitoring/recording yet.  Also, even though the MAC address shows up correctly in CDA for their extension, it's important to make sure the extension is only used once in CUCM by searching the route plan report.  Otherwise, the VoIP Monitoring service may receive the wrong device's MAC address when trying to filter for the RTP stream.

Make sure the PostInstall tool on the agent PC has the correct NIC selected to capture traffic on.  This is important when the agent PC has more than one NIC installed.  This utility is also in C:\Program Files\cisco\desktop\bin on the agent PC.

If the phone seems OK but the problem remains, you can isolate it to a PC by connecting the agent's phone to another PC, like the PC of an agent that can be monitored.  If the non-working agent can be monitored using their own login to CAD and their own phone on the other agent's PC, then it's just a matter of finding differences between the two PCs that can lead to the problem.

Thanks,

Brendan