Correct placement of the GNSS antenna is crucial to the quality of the data recorded by your VBOX Mini Pit Lane Timer. You can read more about where to place the GNSS antennas here.
If your VBOX Mini LapTimer is struggling to acquire a satellite lock, you may have to perform a GPS coldstart. This may be necessary if the unit has not been in use for a number of days or it has moved location dramatically since its last use. You can perform a coldstart by selecting MENU > GNSS OPTIONS > COLDSTART.