@Federico
I had the same problems with sleep recording. It started to work about one week after I enabled the 24/7 HR measurement and sleep recording.
No idea about VO2 max, though. I suppose it also depends on the sport you practice and the HR max you reach and for how long. For example, VO2 won’t be recorded if you do walking or other low HR activities.
Another solution is just to change watch. I have had Suunto for 20 years now, and I am really fed up with them. I gave back last year 1 Suunto 9 and 1 Suunto 9 plus because of quite unreliable HR measurements and totally crazy open water swimming GPS tracks.
The last decent Suunto was the Ambit 3 Peak, which I still have. A. bit outdated, bulky and very uncomfortable to wear, however reliable and with decent measurements.