In most of the cases, it is probably because you are not currently in the area bounded in the map. The area is determined by the placement of mapping points for geolocation and should cover the area illustrated in the map.
If this is your map and you are currently in the area depicted, make sure to verify there are mapping points placed in the “corners” of your map. This will widen the bounding area and detect your location within it more accurately.
For more information about mapping, be sure to explore these guides: