No Sound for TV apps thru Marantz AVR


I recently bought a Marantz SR7012 from Accessories4Less as people here recommended them.

Anyway - I also have a new LG OLED 55 inch TV and when I try to play Netflix and You Tube apps I am not getting sound from the AVR. What is strange is that it works intermittently and only for some movies and videos - but bottom line it is not consistently working at all and rarely thru all 5 speakers.

Accessories4Less says itsthe TV signal that is the problem and LG says my setup is correct and that it is the AVR that is the problem. 

I have it set up with both ARC and Optical cable .... Any suggestions asside from returning the AVR and trying another one ?
128x128pianosteve22
HDMI ARC is one of the most unreliable technologies in existance. Optical should work though, make sure your Marantz is set to the right input and make sure the tv is outputting sound through optical by checking it’s audio settings.
Thank you  - 
I adjusted my setting as suggested - and things are working better, but the youtube videos (played on the LG TV) are only being processed by my AVR in stereo through my front 2 speakers. 
I would like to hear the youtube videos through all 5 speakers.
Is there a way to accomplish this ?

Steve
First go to your Youtube settings and make sure is is broadcasting in multichannel.  Then do the same in the LG.

Bill