I think the exercise ring only registers if you are doing heavy enough exertion for it to consider it a work-out.I think it's buggy. I get up to get water and go to the bathroom multiple times in the night but don't always get credit. Also, the exercise ring is complete BS. I've gone on 5 mile hikes and/or walks and not closed the exercise ring.
i.e., casual walking, etc. will not trigger it, but a proper workout where your pulse rate is elevated "enough" will do so.
Normal walking counts for move ring, but not as a "work-out" (exercise ring) unless you're pretty brisk about it and get your pulse up. i.e., distance doesn't count, level of exertion does.