The modern world is an increasingly noisy place and finding a quiet place is becoming harder and harder to find. The most likely reason your sound level meter won't read below 40 dB is simply because it is not that quiet. There are enough background sounds that 40 dB is the sound "floor" where you are reading.
The National Parks Service commissioned a study to predict the typical sound level during a summer day across the US. The results of that study can be found in these Science News and Scientific American articles. As you can see on the map, in most developed regions of the country, the average sound level is at least 45 dB.
Of course, there is also the possibility that your sensor isn't operating correctly. First, check that the sensor correctly auto-ID's when plugged into an interface. Second, check larger sound levels. Does normal conversation, when the sensor is about an arm's length away, read between 55 - 65 dB? On the sidewalk next to a busy street, does it read 70 - 80 dB? If your sensor does not appear to be reading correctly, contact Vernier tech support.