I just get my watch yesterday and my AOD function is not work too.
Then I found that I am not the only people facing the same problem and some solutions are listed by others. Such as:
1. Reset your watch and reinstall the app in phone
2. Turn off the heartrate sensor
3. Turn off therater mode, night mode
All above solutions I tried are not work for me. And then I found that AOD is not work when I wear it. It works well when it is idle. The watch fall asleep when I wake up the watch by wake up gesture.
So, please try to turn off the wake up gesture (motion and touch), the watch now will not wakeup when you raise your hand or touch it, but the screen is still on as AOD is enabled.
The screen will return to AOD mode after you use it according to the screen light time you set.
Please try it, it works for me! I hope it can help.