iPhone 8 screen will not turn off automatically when not being used

Hello and welcome to Apple Support Communities, derekfromcoleraine.
If I understand your post correctly, you have to manually press the Sleep/Wake button to lock your iPhone as it is not automatically locking. I'd like to help.
Let's take a look at the setting that can affect this feature. Tap Settings > Display & Brightness > Auto-Lock and see which amount of time it is currently set to Auto-Lock after. If you have Low Power Mode enabled, Auto-Lock will be automatically set for 30 seconds. For more information about Low Power Mode, see this article:
Use Low Power Mode to extend battery life on your iPhone
Let us know if that resolves this issue for you.
Thanks for using the Apple Support Communities. Take care.

Lots of people have this problem, if you go to Settings > Display and Brightness > Auto Lock and set a time. Hope this helps!

I think I figured out the permanent fix. Change your wallpaper. I noticed I had a picture on the “locked” and “home” screen that had minor animation. I changed it to a plain/static picture, and my phone goes to sleep mode perfectly, now!

My wife's phone had the same issue and the old standby of turning it off and back on again fixed it too.

I too have found that 11.1.2 seems to have sorted the problem as it’s been 2+ weeks on the phone as well.

For this having problems, ensure you install that and also set the timeout value again (possibly to a different value initially- as that’s what I did).

@nstovell - no need to reboot phone - just terminate the apps with a problem. Not perfect but a bit quicker than reboot.

Restarting the iPhone 8 worked for me. Hopefully, it stays that way.

I was having the same issue and the restart worked for me as well.

Try installing the latest update, for me it worked

I did a hard restart, and it worked for me. Let's see if it comes back as it happened to others.

Seems like the latest OS update (11.1.2) has fixed the issue. I'm a week in and it seems to be working....for now.

I have this issue as well, I have to restart it about every 10 days to get the function back. It's very annoying considering the cost of the phone and the fact that it's obviously not an isolated incident.

At this point, I haven't seen the problem in a long time. I've done the following since "the fix", so who know which of these has taken care of it.

1. Restarted the phone (hold power button until power off slider appears, wait a few secs, press power to turn back on)
2. Updated from 11.0.2 to 11.0.3.
3. Updated from 11.0.3 to 11.1.
4. Updated from 11.1 to 11.1.1.

For those who are still experiencing this problem, it's probably worth getting updated and restarting. If you still see a problem after that, report the bug to https://bugreport.apple.com. It's fairly useless to complain to Apple in the forums.

Has anyone seen these instruction? Ive been doing the press and hold sleep/wake button until i see the slide button to restart it. I just tried this and it went straight to rebooting, so rather handy.

How to force restart iPhone 8

To force restart (hard reset) the iPhone 8, press and release the Volume Up button, press and release the Volume Down button, and press and hold the Side button until your device reboots, then release. All three button presses should be done in relatively quick succession. After holding the Side button for about ten seconds, the iPhone will force restart.

Just to update my previous post.

I did the hard restart two days ago, and it is still working correctly.

You can try it.