Go to CPPM => Administration => Server Manager => Server Configuration
Select the server from which you are sending the COA and click "Collect Logs" at the bottom.
This will open a new window in which you have an option called : Capture network packets Duration of dump
You can leave it at 60sec. Tick only this option and start it, meanwhile go to the access tracker and send a COA to your device's MAC.
Now click finish and download the Logs. Within this archive you will find a packetdump.cap file and you need to open it with wireshark.
In wireshark enter the following filter : udp.port==3799
We are interested in the controller response, so you need to add a filter for source IP : ip.src==YourControllerIPAddress
Resulting in : udp.port==3799 && ip.src==YourControllerIPAddress
What response are you getting from the controller ?