Diagnosis
Probably not a defect. The 50A6K has basic HDR support (HDR10/HLG), but its peak brightness and local dimming are limited. Incorrect settings can make HDR content look worse than standard HD.
How to Fix It
First, make sure you're actually triggering HDR. Play an HDR show on Netflix (look for the 'HDR' or 'Dolby Vision' logo). Then, press the 'Settings' button on the remote while it's playing. Go to Picture Mode and change it from whatever it's on to 'HDR Theater' or 'HDR Vivid'. These are the factory-tuned modes for HDR. Crucially, go into the Advanced Picture settings and ensure 'Backlight' is set to 100, 'Contrast' is high (90-95), and 'Local Dimming' is set to 'High'. Turn 'Dynamic Contrast' OFF.
If That Didn't Work
The TV might be in an incorrect HDMI format for the port your device is using. Go to Settings > Device Preferences > Inputs, select the HDMI port for your streaming device, and ensure 'HDMI Format' is set to 'Enhanced Format'. Standard mode can't carry the HDR signal properly.
Heads Up
This is an entry-level 4K TV. Don't expect eye-searing HDR brightness. In a bright room, HDR will often look dim. For the best picture, watch in a dark or dimly lit room. Also, the 'Filmmaker Mode' disables most processing but can make HDR look particularly dark on this model.