The LG ARC implementation has driven me a bit crazy as well. Try power cycling the TV and receiver several times, even unplugging them if it doesn't work. Always use the TV remote to turn on the whole set up - the TV must turn on the receiver or it won't work. Also, most receivers have a setting for CEC control and a separate one for ARC; make sure both are configured correctly. With my TV, sometimes the ARC setting resets to TV speakers, so make sure that doesn't happen, too.
Preamp/Amps: Anthem AVM60, Emotiva XPA-2, XPA-5, A-300
Speakers: (7.1.2): Klipsch Reference Premiere / Power Sound Audio S3000i sub
Video: LG OLED65B7A, Oppo UDP-203, Roku Premiere+, Nvidia Shield TV
Room: GIK Acoustics treatments