Low tVOCs levels can occur for various reasons:
You actually have low tVOC levels in the air.
Your device has recently been restarted (unplugged or after a power cut). In this case, the sensors need a little time to re-adapt. This can take up to one week.
Your device has been turned on and off in environments with different humidity and temperature. If so, the sensor needs a bit of time to re-adapt. This can take up to one week.