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Make sure the HR sensor is free of debris / clean. The AOD won't launch if the watch doesn't detect it's being worn.
Please check if the theatre mode is turned off. I had the same issue when theatre mode was accidentally on. Turning it off solved it.
After trying all of the numerous fixes suggested, found out that if you have wake gesture turned on it disables the screen from staying on and showing the whole watch face. But even then it only stays on for max of 5 minutes.