By 'checked with Wireshark' I assume you've ran Wireshark on the PC and you could see RTP / Skinny traffic ?
That's indeed typically the biggest hurdle, either due to the PC not being daisy chained to the phone and/or NICs that do not support monitor mode.
Some other causes that you may want to check to could 'pop up' without making any real changes :
1) If the PCs are laptops with wireless NICs or have a 2nd NIC for some other reason you'll want to check from Network Connections - Advanced - Advanced Settings that the NIC connected to the phone is on top, if it isn't, CTI OS SM service will be listening on another IP than is advertised to the Supervisor client;
2) Some firewall / AV software causes WinPCAP to not be able to switch the NIC into monitor mode, so try to disable that;
3) If you tested with other user ids than the agents actually use, make sure the agents have enough permissions to set their NICs in monitor mode.
Mind you, step 1 can also apply to your supervisor PC, i.e. if you've got a wireless NIC or VPN adapter active and prioritized over whatever interface you use to reach the CTI OS server that can cause the same problem.
Cheers,
Kris