Vo2 Max: after I run, it tells me my status is either ‘maintaining’ or ‘unproductive.’ Despite regular exercise, my fitness apparently rarely improves and if my watch is to be believed actually deteriorates! Please explain?

One important thing to know is that the training status is based on your training over an extended time period. 

https://support.garmin.com/en-GB/?faq=VxKazDQ2mkAmDoQbJriEBA

Look at your VO2 max and training load graphs.

This is a really great report that shows the values of interest: https://connect.garmin.com/modern/report/-1/all/last_four_weeks

What does your graph for 7-day load look like? Are you in the green area and are you increasing the load slowly?