Thanks for getting back.
If "now playing" in stopped state releases all keep awake states, then this is obviously not the source of the problem! The network adapter settings on the laptop need to be set to allow wake up of the pc for gizmo to wake up the pc if it is asleep. Could the problem be another computer on the network broadcasting some packet/data (over the network) be preventing the pc running JRiver MC20 from going to sleep?
Really appreciate your help.