Some of the most common reasons for receiving no data/empty responses from your API calls are:
No record in the MarineTraffic database during the defined timespan
You can try to alter the timespan (be careful not to exceed the upper timespan limit of each API call).
No coverage in the area
The vessel(s) sails in an area with limited coverage from terrestrial or Satellite or roaming ais receiving stations
The actions mentioned are applicable to the following API Services:
AIS
Port Calls
Voyage Forecasts
Expected Arrivals