This is a question that seems to be asked continually. You will see the answer to this question linked on every ship tracking page if you click on the "Out Of Range" link.
Online vessel tracking relies on the use of Automatic Identification System (AIS) data. Most modern vessels even today still use VHF, line-of-sight equipment, to transmit their AIS data. This line of sight limitation will require even the tallest of vessels to be within 80-100 miles of shore in order for the data to be received. Smaller ships or older ships with weak transmitters will transmit significantly less than 80-100 miles. Newer satellite relay services are available (S-AIS) but at a very high cost. The land based VHF relaying still remains a free service for all vessels which is why it is still popular today.
For example see this image below of the Gulf of Mexico:
This is a rough estimate of what can be expected for ship tracking within the Gulf of Mexico.
The area contained within yellow is "Out of Range" for the AIS VHF land based reporting which is used by most ships. The only service that would cover a vessel in that area would be satellite tracking (S-AIS) and as mentioned above most vessels do not use the satellite service. You will also see some circular areas within the yellow area. These circular areas are VHF reporting stations (typically land based) located on a few oil platforms in the Gulf of Mexico. Depending on the height of the AIS antennas on the platform they may be able to pick up ship traffic anywhere from 20-100 miles out from the platform. There are more oil platforms with reporting stations than the three pictured above but the majority of them do not have AIS reporting equipment, they'll use other services like radar to direct ship traffic around the platform.
In conclusion, if the ship you are trying to track is more than 80-100 miles from shore it will typically not show up on the tracking map because it is "Out of Range" from any land based reporting station. There are a few exceptions: areas around oil platforms with AIS relays, ships that use satellite (S-AIS) relay services - but in general if a ship isn't within about 80 miles of shore or less you will not be able to track it live. We do have the option on all of our tracking pages to also select/view "Last Recorded Position". When this option is selected the map will show the ships location speed and track just before it went out of AIS coverage, or turned off its AIS equipment (which happens occasionally when in port).
Hope this helps to explain why you can't always track a ship live.
For additional information: http://en.wikipedia.org/wiki/Automat...ication_System