All GPS recievers take time after a cold start to acquire satellites. It takes 6 minutes to download the constellation ephemeris (satellite orbits) so it knows which satellites are where and which to look for. If the Nav info menu (the one with the hemisphere, satellites, lat, long, alt, etc) shows green satellites, then its just a matter of a few minutes before it all comes together and it gets a fix. Once it gets a 2D fix (it takes at least 3 satellites for this) then it should update your position. The LAX position is the default.
There are at least 12 satellites in a hemispherical view at any given time. The roof blocks the signal from at least 1/4 of them, but fortunately, you only need 4 for a 3D fix, and 3 for a 2D fix (and only 1 for time-tick). This is why a 12 channel reciever isn't twice as accurate as a 6 channel one.
You only have to do this once, as after it has the ephemeris in memory and you're current position, it locates satellites almost immediately (called a warm start). If it doesn't, you need to check your GPS antenna, and the VSS input. GPS doesn't work very well indoors (in a garage), so you'll simply need to drive around in Nav mode for a short while.
If the VSS input is bad, you'll get you're last GPS position, but it won't move on the map.