There are many possibilities for this.

1. You do not have a clear sky-view (Clouds/Rain/Snow/Buildings/Trees can block the satellite connections!)
2. AndNav might be using your last known location (if you have been to the 'wrong' place before)
3. The GPS-receiver of your device is broken

Please check if the the following icon in your statusbar is blinking:

That blinking icon means, the your device is trying to establish a connection to the GPS-satellites, what may take up to a minute with a clear sky-view. As long as the icons is blinking AndNav is not able locate your current location (where it may use your last known location!)

AndNav¹:
As soon as a connection is established, you can manually make AndNav¹ recalculate the route, by opening the menu and pressing the -item or by pressing the 'R'-key on your keyboard.

AndNav2:
As soon as a connection is established, AndNav2 will automatically recognize that you are off-route and recalculate the route after ~5 seconds.