Intensity minutes on vivoactive 5 too low

I have a vivoactive 5.

I have been increasing my speed on the treadmill and have been spending more time in zones three and four and yet the intensity minutes are lower than when I spend more time in zones one and two. 

This has happened the last two workouts and I've only had the device for 2 weeks. I certainly don't want to keep a pricey device if after only 2 weeks, I'm intensifying my workouts, the intensity minutes aren't being recorded properly. 

Anyone else having a similar situation? 

Thank you!