I have a Samsung 65inch 4k TV, model UA65MU6880.  It has 3 HDMI input, and HDMI 3 is HDMI ARC.

I have been using all 3 HDMI input at the same time.  HDMI 1 and 2 connected to a TV box and a laptop, and HDMI 3 (ARC) connected to a soundbar.  

But recently when I tried new cable with HDMI 1 and 2 (did not touch the HDMI 3 at all), something weird happened.  Now the HDMI 3 (ARC) can only recognize the soundbar if no other device is connected to HDMI 1 or 2.  Otherwise, the HDMI 3 (ARC) cannot recognize the soundbar and sound can only come out of the built-in speaker.   Which effectively mean, I cannot use the soundbar when using the tv box or the laptop, but can only use the soundbar if I use the TV built-in apps like Netflix/apple TV.  

I tried resetting the TV - did not work.  Tried calling the customer service - asked me to unplug the TV for 15 minutes (says the circuit board will reboot/reset or something) - did not work either.

Anyone has encountered this similar situation, or have some ideas as to what might help? thanks.