Step 1) Make sure feature is turned on, go to "Settings > DEVICE > Gestures > KNOCKON > Screen on/off" and turn on the feature if box is not checked.
Step 2) Next, go to "Settings > DEVICE > Gestures > SENSOR > Motion sensor calibration > Calibrate sensor" and follow instructions to calibrate the sensor. If you still can not fix the issue, go to Step 3.
Step 3) If KnockOn still does not work, type the code according to your variant below in dialer for hidden menu access:
3845#*800# (AT&T)
3845#*803# (Canadian)
3845#*802# (International)
5689#*980# (Sprint)
3845#*801# (T-Mobile)
3845#*980# (Verizon)
For Sprint LS980 when hidden menu shows up, go to "Device Test" then scroll down to "Set Proximity Limit" and then "550". Tap "Ok" and now hit "EXIT". Tap back to get out of menu and reboot.
For all others when hidden menu shows up, go to "Settings"and then go to "Update Touch Firmware". Wait 5 seconds for touchscreen to work again. Exit the menu and reboot.