Intensity minutes problem

1. In the Garmin Connect App - go to More > Garmin Devices > Tap on the listing for your Venu 3
2. In the settings screen choose Activity Tracking > Weekly Intensity Minutes 
   1. Is "Use Heart Rate Zones" enabled?
   2. What Zone is selected for Moderate Intensity?
   3. What Zone is selected for Vigorous Intensity?
3. In the settings screen choose User Profile > Heart Rate
4. Make note of the Max HR shown and how the percentages/numbers are applied to each zone

Now that you have that information accessible, take a look at How Are Intensity Minutes Earned?

During the time period you feel you should be getting Intensity minutes, is your heart rate within the zones specified for Moderate or Vigorous Intensity?