Calibration can fail for a number of reasons. The most common being either the humidity failed to rise while inside the calibration kit or too much fluctuation occurred in humidity or temperature while inside the calibration kit. 

The first step after a user has failed a calibration is to attempt to calibrate the sensor again. When done correctly, the calibration process rarely ever fails twice.

If the calibration process continues to fail multiple times, these tips can help!

1. Ensure the sensor is in calibration kit
2. Ensure that a fresh kit is being used, calibration kits do not last forever
3. Ensure the calibration kit sealed completely
4. Avoid temperature fluctuations (often even air/heat systems which cycle or direct sunlight can cause calibration to fail)
5. Ensure the calibration kit is away from windows/doors/temp or humidity fluctuation
6. Its important to remember that during calibration, even small fluctuations can cause issues