The GPS inside the device needs a good reception of signal from satellites in order to provide an accurate positioning of the boat. Any metal objects located near the device block the GPS signal reducing the location accuracy. This may result in "No GPS position" or tracks going straight instead of following the boats true path.
Resolving the issue:
- Improve the device position
- Make sure that it is oriented with cables facing downwards
- Make sure that there are no metal or conductive objects near and especially above the device
- Make sure the device is installed above the water line
Checking if the issue is resolved:
After you have made the adjustments, toggle bilge or door or shore input to force the device to update the status to server
- Open Sentinel app and select your boat
- In the sensors screen check the GPS signal indicator. At least 2 bars should be present.. 
- In the Web app (web.sentinelmarine.net) you can see the history of the signal by clicking on the signal indicator. Measurement must be above 10 for reliable tracking, recommended is above 13.