The face of the SLR camera is close to the screen, and the external flash is not bright.

First of all, it has nothing to do with whether the external flash flashes when the face of the SLR camera is close to the screen!

After installing an external flash on the camera, you must turn on the automatic flash switch before it will work. It is somewhat similar to the flash settings on the camera: automatic flash, no flash, forced flash and delayed flash. Of course, if you don't choose external flash to flash automatically or forcibly, you won't flash, hehe!