Diagnosis
The TV's energy-saving or ambient light sensor is aggressively throttling the backlight brightness.
How to Fix It
Press the Settings button, go to All Settings > Picture > Advanced Settings > Energy Saving. Set it to 'Off'. Immediately check your picture mode (like Standard or Vivid) and manually adjust the 'Brightness' and 'Backlight' (or 'OLED Pixel Brightness') sliders up.
If That Didn't Work
Check if 'Eye Comfort Mode' is on. Go to All Settings > Picture > Additional Settings > Eye Comfort Mode and disable it. This feature adds a warm, dim filter that can't be adjusted.
Heads Up
The 'Filmmaker Mode' and certain ISF presets are calibrated for accuracy in dark rooms, not brightness. Avoid them if you want a punchy, bright image for daytime viewing.