Stu,
As Ian says, to get the Mode-S/3D Data you need to be within range of a PAW-OGN-R Ground Rebroadcast Station. Also to prevent Ground Stations cluttering the frequency by continuously transmitting positions for all aircraft within their range, you need to be receiving the Mode S aircraft’s transmissions directly on your PilotAware so that it can identify the aircraft you require a position for (by that aircraft’s ICAO Hex ID) and ask the Ground Station for position data for that specific aircraft.
Hope this helps.
Regards
Peter