Camera flash drains the battery. To save battery power, the camera disables the flash (even when you have set the flash mode to auto flash or flash on) when one of the following occurs:
You're on a phone call.
The flash will be disabled temporarily when you're on a phone call. After the call ends, you’ll be able to use the flash again.
Battery level reaches 20% or lower.
Recharge the battery so that the camera can have enough battery power to use the flash.
Cold weather may lower battery performance which affects the flash.
To make sure the camera has enough battery life to power up the flash, keep your phone warm. When not in use, put the phone inside your jacket to keep it warm, and try to avoid sudden temperature changes that may affect battery performance.