Number one, make sure your LG TV software is up to date.  If that doesn’t work, unplug the TV from the wall and wait 5 minutes.  Then plug it back in.  This will trigger an HDMI handshake, and sometimes that is all an out of sync HDMI connection needs. 

If a hdmi handshake doesn’t work:

Has it ever been in sync with Atmos content or has it always been out of sync?

Is the delay for all atmos content played using both the LG TV apps and external devices plugged into the TV?

Just with atmos content played using the LG TV apps?

Just with atmos content on external devices connected to the TV?

The Sonos Arc will play the Atmos content as fast as it receives it and with the audio being behind the picture there isn’t anything the Arc can do delay wise.

I think some LG TVs still have negative and positive adjustment in their av sync settings, but getting it set right for Atmos, positive, which actually delays the video frames, might mean the audio is then out of sync for non-Atmos content 

If it’s from content played from an external device like a set top box or Apple TV or Firestick, ensure your TV’s sound setting is set to Pass-through, if that option is available (check the TV user guide), so that the TV does no processing to the sound before it reaches the Arc. 
Also ensure you have any Atmos setting turned on in the TV’s sound setting if applicable (I have a Samsung which needs to have Dolby Atmos set to On…).

Also, I’ve read that eARC makes all the difference for audio sync. Your TV doesn’t have that you say, although it might not be the issue. (My Arc is connected to the Samsung HDMI eARC port and doesn’t have any lag on external devices (on pass-through)…)

Thanks. The handshake seems to have helped a bit.

it seemed that it varies between sources and probably better from native TV apps.

i prefer to watch most of my stuff through Apple TV 4K. It was playing fine from Apple TV films I’ve bought but was getting worse. Netflix was way out.

I’ll have to sit down and monitor them all one by one.

No option for pass through I can see and the Atmos Google switch is greyed out when HDMI as the output is set.

It makes sense the native TV apps would behave better, as the TV has more direct control of the video and audio timings and delays to automatically apply. It can also stream the DD+/Atmos audio direct to the Sonos Arc

For the Apple Tv, it’s more complicated.

Is the Sonos Arc definitely receiving Atmos when using the Apple TV, or is it just Dolby Digital 5.1?

Apple TV doesn’t support bitstreaming Atmos and when set to Auto best available will usually convert Atmos to Dolby MAT if the audio device is on a hdmi 2.x or hdmi-earc connection.

If the audio device is on a hdmi-arc connection, then as far as I know, there isn’t enough bandwidth so it would send Dolby Digital 5.1

If the Sonos Arc is showing Atmos as the audio type when using the AppleTV, then the TV is likely not set for pass through, hides the Sonos Arc from the Apple TV and informs the AppleTV it supports DD+/Atmos. Apple TV sends Dolby MAT which the TV is then converting from into DD+/Atmos which fits inside hdmi-arc bandwidth.

The TV audio decode and encode is a potential point where the Audio and Video drop out of sync, depending on the LG software.

Search ‘LG B7 Sync Atmos’ on this forum and there are a few results - some I believe discussing the same issues and/or whether you can achieve real Atmos from the B7 without eARC. 

I wonder if any of these threads are helpful to you…?