Mine was recording fine until I had to do a restore to get my GPS and other data back in sync.
It stopped recording at that point.
I have got it back working it would seem by resetting the fitness calibration data
From your iPhone>Watch>Privacy>Reset Fitness Calibration Data>Tap this
You will just get a confirmation box that tells you that your metrics will not be as good if you do this.
Tap confirm and do it. You will get nothing else, no confirmation but it is done.
Then go for a 20 minute outdoor run or walk using the workout app at your normal speed on a flat road/path etc
This will re-calibrate the watch.
The next time you do a workout (not this time) it should show your V02 max
That is what worked for me.