I've found that on my 1520.3 I need a certain rhythm, a specific cadence to get double tap to work.
When you say the "sensor light not blinking" that is not a correct idea. There are sensors and LED lights. You have sensors, they do not light up. There is no LED light to create a blinking notification with. That's Android. Lumias have Glance screen, Live Tiles, Notification Center and notifications pushed to Lock Screen... No little blinky light. It's just not part of the hardware.