Cardio Fitness Trends - V02MAX

I have been monitoring Trends, specifically V02MAX in my Fitness app. I'm using an Apple watch series 8 and have been doing a variety of activities. It tracked my VO2 all year but stopped in Nov/Dec and has just started working again. There was no major changes to the activities that I have been doing. I have all my activities saved and can see them in the health app but they have not been pull through to the VO2 data. Can I manually add the data? Any one hear of any issues with the App in Nov/ Dec. Its very frustrating as I was hoping to see an increase in the VO2 for this period. Thanks