sim808 GNSS location update
That's just how GPS works. Once you've locked to a position, the internal routines will have it easier to keep your position correct. As soon as you start moving, things get more complicated for GPS receivers.

Now, walking from a flat to a car shouldn't be much of a challenge. Check your antenna, the surroundings (is this an urban scenario where you in the middle of building canyons that simply not allow for reception?) and whether you simply need a better receiver.