I had this problem too, for the last week and was considering buying a new watch since I couldn’t see the screen at all when outside which is where I use it most. Luckily however, I did some research into this problem on youtube and the only issue was my brightness was set to dim, rather than normal or auto, which can be seen outside in direct sunlight. I didn’t even know you could change the brightness setting ahah. To change the setting I scrolled right to the last page of icons of my Fitbit Charge 3, clicked settings and that is where I found the brightness setting. Simply tap brightness to change it. Hope this helps you!