Why doesn't VO2 max change ?

I found something here that I haven't seen before: https://support.garmin.com/en-US/?faq=lWqSVlq3w76z5WoihLy5f8 

VO2 Max Estimate From All-Day Heart Rate
Newer watches are capable of generating VO2 max from all-day heart rate data. VO2 max generated from all-day heart rate will only show if you have not generated VO2 max from another source above, unless the all-day heart rate VO2 max is better than VO2 max generated from other sources.

Thanks e7andy.

According to this support, I see

If you are having difficulty obtaining a cycling VO2 max estimate, try riding a course where you can maintain a steady effort. You can also try using an indoor trainer.

• Cycling activity must be 20 minutes in length or longer without stopping (OK. Each journey I usually make from 1h-1h30 minutes. Weekends can be >2h)
• Heart rate data from either a built-in optical heart rate sensor or from a chest strap (OK. I use HRM )
• Heart rate must be elevated to at least 70% of your maximum heart rate for at least 20 minutes continuously (I set my max heart rate for cycling around 170. Maybe I haven't hit this target yet ? )
• Power meter is required ( I do not use)

Since you don't have a power meter you will not get any cycling VO2 max values not matter what you do.

If you want to record an accurate vo2max you will need a power meter (watch doesn't know if you are in a group drafting, head or tailwind, riding gravel, mtb tires, aero TT bike, etc... speed on bike is worthless for effort).  

If you want to update the green line (running/walking/all-day) to an accurate actual reading... I would recommend working your way up to a 15-20min jog/run on flat terrain in mild temperatures.  It will slowly shift to your actual vo2max if it is important.  If you are not a runner, don't be discouraged at first, running economy/efficiency takes time.  Within a few weeks even with minimal fitness gains your pace at the same HR/effort will improve.