G'day

The problem that you describe is not uncommon with most GPS or plotter devices. These "domestic" devices have an accuracy of somewhere between +/- 3 metres and +/- 5 metres. When drifting very slowly, this small amount of error may mean that the plotter does not update itself because it "thinks" that it is still in the same place. It is only once the plotter moves outside of the area of error that it will then update itself. It is for the same reason that people relying on very accurate locations have to use a very expensive device known as a Differental GPS.

It is unlikely that your trolling motor head would be physically blocking out sufficient satellites from the "view" of your Plotter's antenna, but I wouldn't really be able to tell without seeing a picture of how you have the unit mounted. You can check whether the unit is receiving enough satellites to obtain a "fix" by going into the "System" menu and then select the "GPS Status" menu. I doubt that electrical interference from the trolling motor would be an issue. GPS/Plotter units are generally quite resistant to electrical noise.

If you are homing in on a brush pile or other structure that you have coordinates for, then as you begin to slow your speed across the water it is often useful to have the unit in split screen mode so that you can watch the sonar simultaneously with watching the plotter.

I hope this helps.

Cheers