Same issue here, Soundbar HW-K450 and TV 55NU7402 with latest firmware on both installed.
Before buying the NU7402 this week I used to have an MU6102 with latest firmware installed and had no issue.
SInce buying this NEWER model this issue appeared, tried pairing/unpairing/everything, no result.
This just worked me and I found it by accident.
I have a HW K450 soundbar to Samsung 6400W TV
The issue...TV wouldn't recognize SB via Bluetooth!
1. Power on SB and use source button to set to BLUE TOOTH READY.
2. HOLD source button on remote for a few seconds until SOUND CONNECT SEARCH TV appears!!!
THIS WAS THE KEY!!
3. Search TV - Sound - bluetooth - scan
4. TV promts a device has been located and CONNECT soundbar!
Make sure bluetooth button on remote is engaged to ON for the above process.
Also ensure in SOUND on your TV that in extra settings that ADD NEW DEVICE is listed to ON to receive your soundbar's request to pair!
If you want initialize SB, power OFF and hold pause/play button for 5 seconds. Done.
I've had numerous connection issues which I've solved through all the above mentioned advice but stumbled upon point 2 above which was a defining factor this time, nothing else worked that previously had fixed the issue!
Happy to share my experience after 3 hours!!!!