Do you have adaptive brightness turned on in your settings?
I do, but the problem is that it doesn't brighten again when the lighting changes.
Like if I'm doing something, and walk through a dark room into a brighter one, it'll dim in the dark room, and I won't be able to see it when I come back into the light.
Be sure that you are not blocking the light sensor near the top of the phone screen. If you do, the phone thinks that it is dark and will continue to use the dim screen.