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 ?
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 ?

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

YouTube is stereo only (expect some of their YouTube Premium/Red content).

To do pseudo-surround, switch the mode on the receiver from Stereo/Direct/etc. to Dolby Pro Logic II (or Dolby Surround if yours is newer).

It may be considered a sin here, but I listen to most all my music upmixed to surround (music mode, where vocals are duplicated to the center instead of extracted), and unlike All Channel Stereo, the surrounds only play the instrumental, and at a low volume so adds ambience without being distracting; I believe Dolby Surround doesn’t have separate modes, but you should still be able to change Center Width/Spread to adjust how it handles vocals, at least those that are meant to be centered).
Actually i found the setting on the Marantz AVR that allows me to play the You Tube music from the TV in either Dolby Surround or in Stereo. 

Thx for your help ....

and,it is probably your Receiver .Optical gets you going but,it would be nice to have the ARC working properly.I had to use Optical on My older Onkyo[TX-NR838] because it was hit or miss on ARC