Lumia 520 does not have a Flash for the rear camera... Thus, no light can be lit on the rear of the phone for use as a flashlight...
Some Flashlight programs do allow the screen to be used as a flashlight by lighting up all of the screen's pixels in white. That is the only type of flashlight functionality you can use on 520. Check your Flashlight App settings for such a screen as flashlight feature and see if it can be enabled as a default. I use this app
Flashlight | Windows Phone Apps+Games Store (United States) It has a screen toggle switch built into the App on the "home page" when you open it and slider controls for brightness and color when the screen is light up for flashlight mode...
