This can happen if the manufacturer has not programmed unique serial numbers into each adapter. If this is the case, then all adapters look the same on the USB bus and we cannot talk to an individual adapter.
If you run the debug tool here, with the adapters connected:
http://kb.displaylink.com/269
Then look in the system details.txt, you will see the adapters currently connected. If only 1 is listed eg:
Nivo's currently attached: PID_0110\\0110-000000
If there is always only 1 shown, and they always have the same id, then this will be the problem.
Thanks
Wim