Knock on not working, what should I do?

hye i have d802, and after i upgrade from 4.2.2 to 5.0.2... suddenly a knock on feature stop working...i can knock off the phone but no knock on... i tray to "update touch firmware" in hidden menu and nothing append,
i try to downgrade to 4.2.2 and still there this option not working!!!
what should i do?
please help me

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