Here the same problem, I have read something in the garmin connect forum and they say it is connect failure.
They must be looking at it to solve it.
We will have to wait.
I wish I knew how it decides to calculate a value. I mainly ride and I don't have a power crank or anything, so I know I don't get a cycling VO2 max (yet!) but I do get a VO2 showing up as a graph some weeks. Has been saying no data (and no graph) plot for last month. Even though riding regularly.
I don't know if the general VO2 includes the cycling but is just general or its picking it up from just the odd run, recorded works, etc! I tried searching but it knocks out the words like VO2. Doh.