Apple Watch Vo2 Max not generating

Long time Apple Watch user - issue with Cardio Fitness not generating. Have taken the minimum # of specific workout types in excess of the 20 minute threshold (some with the iphone with me and some with only watch). 20+ outdoor runs and walks within the last couple weeks.

never had an issue with cardio fitness generating on my older watches. Ultra 2 is my model.

have gone mad reading the old forums to no avail…. I receive all other data crystal clear (sleep, other HR readings, steps, etc.) Thanks in advance!