I have a SONY XPERIA Z5 Premium . The problem is the tiny LED notification light doesn't show red light. i have tried various led light management applications from the play store but still ain't working. What may be causing this? Running Android 6.0 Marshmallow
Have you tested the LED through the service menu?
To enter the service menu go to the dialer and paste in the following code
The LED should start to blink in green then blue then red. If your led is not blinking red I would suggest your LED is broken.