Flash light seldomly works

Your phones flash by default is controlled by the lighting the camera see's so it may think it's not need. As an example someone is standing by a southern window where the sunlight is spilling in quite intensely. Being back lit the camera's sensor won't fire the flash to fill in. Here you need to open the camera app and manually tell the camera to fire it.

The second use of the flash is as a flashlight to help you find your way walking or looking at a menu in a dark restaurant. The flashlight app is where you can control it.

If this doesn't answer your question you'll need to explain in more detail what the problem is.