Try to position the GPS where it will have the clearest views of the horizon to minimize signal blockage from your vehicle.
Try another GPS receiver too. I have a Garmin 35 and it works really well. The Earthmates are often thought of as crap. I have my Garmin located in the back window and it works fine.
Iíve also found crappy GPS software can also cause you to miss turns. I tried a few and settled with CoPilot.