DIAGNOSIS: The TV isn't switching properly into its proper HDR picture mode, or the console/PC is sending a mismatched signal. This model has specific HDR settings that are separate from SDR.
FIX: When your HDR game or content is active, press the home button and go to Settings > Picture > Expert Settings. Make sure 'Contrast Enhancer' is OFF and 'HDR Tone Mapping' is set to 'Static' (not 'Active'). Also, in the main Picture Mode, select 'Game Mode' if you're gaming—it has its own HDR calibration. Then, on your game console (like PS5 or Xbox), run the built-in HDR calibration tool from its system settings.
IF THAT DIDN'T WORK: Check your HDMI port and cable. You must be using the HDMI 2.0/HDCP 2.2 capable port (usually HDMI 1) and a certified High-Speed HDMI cable. Also, on the TV, go to Settings > General > External Device Manager > HDMI Signal Format and set that port to 'Enhanced' (or 'Input Signal Plus' ON).
HEADS UP: In a bright room, HDR will naturally look dimmer because it's preserving highlights. This is normal. For a brighter image, you can adjust the 'Shadow Detail' and 'Peak Brightness' in the Game Mode HDR settings, but you'll lose some detail.