Karinita, I, too, have suddenly had my intensity minutes stop registering. Generally, Garmin appears to be having too many issues to be reliable nowadays!
I'm having the same issue here too. None of my exercise is being transferred into the intensity minutes. Heart rate and steps are recording accurately but intensity minutes are still at 0 after 4 runs…
‘Don’t see how it could be continually recording HR, but somehow not tight enough for Intensity minutes... It was working fine for a long time, them one day completely stopped recording IM…
I also see this issue on a Vivoactive 4S. I only get credit for Zone 3 minutes and Zone 4 just get ignored instead of counting 2x like they are supposed to! That leads to a pretty silly day where I ride my bike straight up a cliff for hours and Garmin is like "big deal". No answers for you, just keeping the thread alive.
This should help - How Are Intensity Minutes Earned? | vívoactive® 3 | Garmin Customer Support
This fixed it for me, thanks!