I am using BME680 with besc library and while testing I could see that the sensor is callibrating after few hours of sending data. What is the reason for this and Is this normal behaviour ?
What is the time interval you are using in your application when calling the BSEC lib?
The calibration procedure are also depending on the environment gas component changes.
If you have a very stable environment without too much VOC concentration change, the BSEC lib can't find a proper base line, then the calibration is hard to achieve the level 3.
You can try to give some clean air ( bring it to outdoor) or bad air (put VOC source close) to speed up this calibration.