The cover screen on my Z Flip3 won't get very bright. It used to, but I don't know what changed that keeps the screen really dim. The only times I notice it bright are when I first start the phone (the Samsung Galaxy logo appears) or when I use the cover display for taking pictures simultaneously with the main screen. When I launch the camera when the phone is closed and use the cover display as viewfinder, the screen is very dim and barely visible.
