iWatch not measuring vo2 Max

I have iWatch 7 and have been wearing it since November 2021. I run every week, and I am currently training for a marathon but no matter what I do, it won't measure my vo2 max. The watch app is set up properly but when I go into Health and into Cardio Fitness it says that no devices are sharing data. I set it up when I got the watch but it hasn't recorded any data. A few days ago, I tried re-pairing the watch to see if that would make it spring to life, but no luck. My privacy settings are correct and it records all other heart related data. What do I need to do to make it work?