If you have an AISWatchMate 670, be sure you have a GPS connected either directly to a port on the AISWatchMate or routed through an AIS receiver. If you are using a transponder, make sure the transponder is setup to provide GPS data. You can verify that GPS data is being properly received by the AISWatchMate by using the procedure for examining the incoming data stream.
Make sure you've got the port that the GPS is connected setup properly. Use the device settings menu to verify that "GPS" is selected as the input device. However, if the GPS data is being provided by an AIS receiver or transponder then select "AIS" as the input device.
Ensure the GPS has a valid satellite fix. If it doesn't indicate a valid fix then the WatchMate won't use the data it provides. Use the GPS Satellite Status page. You can also verify this by examining the GPS screen or by looking at the data stream on a AISWatchMate 670 or 750. When you are looking at the NMEA data stream, locate the $GPRMC sentence and you should see something that looks like this:
Look for the "A" in the 3rd field. If it is a "V", or something other than "A", then the GPS doesn't have a valid fix. Check for a loose antenna connection or other wiring problem on the GPS.