Could be one of the following problems:
1. Your car's window film contains mercury.  The more percentage of mercury it contains, the more difficult to get a fix on satellite signal.  So you need to use an external antenna.
2. Cloudy sky or heavy rain will cause the weakening in GPS signal.
3. It is difficult to get the satellite fix under tall buildings or trees, car park, tunnels, bridges or highway overpasses.
4. The unit is seldomly use, so the satellite positions in the unit are lost.  It needs to be turned on to get a new fix while remain stationary (depended on the models) and under clear sky.