The performance of your phone's accelerometer deteriorates over time .
So it is very important that calibration is done from time to time.
-> Easy calibration process.
-> Move the red dot on the screen into the green square and click on calibrate.
-> AutoCalibrate option which notifies you when your phone's accelerometer needs calibration.
Free version (with ads) of accelerometer calibration is available here:
NOTE: Works better on rooted device.