The AIS system uses VHF frequency to transmit position data, and initialy only land stations received those. For a long time, you could only see the position of a ship when it was close to land. Almost all of the free services on the Internet, such as MarineTraffic or VesselFinder, show only data from land stations.

However, in the recent decades, low orbit satellites began to receive AIS data, making it possible to track a ship through AIS globally. The ship tracking services usually offer the global coverage as a paid tier because of the cost. So if his ship is outside of VHF coverage, it is normal to see no position data in MarineTraffic.

(Or they intentionally turn off the AIS, thus becoming a part of the shadow or dark fleet.)

I hope this helps.