[Update] Please let us know if this post helps you or not!
How to access hidden menu on LG G2
Some LG G2 Users said that even a hard reset can not fix the “KNOCK ON NOT WORKING” issue.
Make sure feature is turned on, go to settings > general > gestures > knock on
Turn on the Knock on feature.
Try motion sensor calibration, go to the settings, general > gestures > motion sensor calibration.
If you still can not fixed the issue, Go Step 3
If you lost the “knock on” function on your LG G2 after you flashed a custom firmware or made a modification to your system.
Please run an all auto test from the hidden menu on your LG G2:
Dial 3845#*802# -> Device Test -> AATFull (All Auto Test-Full).
After the device test,
Dial 3845#*802# > setting > update touch firmware
Notes, please change the 802 to your LG G2 Model Number.