Ok. You need to give us some more details to be able to help you.
Could you post web links to a few activities that you think should give you a VO2 max value? It is often eaiser to just look at the activities to spot any issues. In the activity the activity type, Garmin device, heart rate, power etc will be visible.
Which Garmin device are you using?
Is it the running/walking or cycling VO2 max that you are trying to get to work?
What are the requested requirements you've met? They are listed here https://support.garmin.com/en-GB/?faq=lWqSVlq3w76z5WoihLy5f8 but it is a good idea to tick each of them off again just to make sure that you really have met all of them for the activity type you are using. The activity type used is very important.
