HTC Detail User Manual page 199

Camera 199
Camera flash doesn't turn on?
Using the flash eats up much battery power. To save battery
power, the camera disables the flash (even when you have set the
camera flash mode to Auto flash or Flash on) when one of the
following occurs:
You have an incoming call
The flash will be disabled temporarily when you have an incoming
call. After the call ends, you'll be able to use the flash again.
If you have set the flash to Flash on, it turns back on after you
answer and end the call.
Your phone's 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.
Portable Wi-Fi Hotspot is turned on
The flash is disabled automatically when you use Portable Wi-
Fi Hotspot to share your phone's data connection. For more
information, see "Portable Wi-Fi Hotspot" in the Internet chapter.