Exercise App problem

Does anyone else have issues with not getting the Exercise Ring to close? I normally walk 60+ mins at a good pace, watch snug on wrist; sometimes the ring closes after 40 mins (it’s set for 30), but other times, it will only give me credit for 6 or 10 or ... mins. Many days I’ll have 5-7 miles of exercise but ring does not close.

What am I doing incorrectly?