Activity minutes not counted

[Bears25]

Earlier this evening I did a small cardio workout that lasted 18 minutes. I was in the fat burn zone for the entire duration, and yet I didn't get any activity minutes for this. Yesterday I did a similar workout and it counted all the minutes as activity minutes. I was in fat burn the entire time, and the workout was only 12 minutes. 

Can anyone tell me why today I get no activity minutes?

[Esya]

This article explains how FitBit calculats the active minutes.

[Bears25]

I get how it tracks them. It just seems inconsistent. One day it counts the activity minutes, the next day you do the same workout and it doesn't count. Like even just this morning I walked to work, took 11 minutes and my average heart rate was 104, it counted it towards my activity minutes. But yesterday my workout was 18 minutes and my average heart rate was 130, and I get zero minutes counted. I worked harder yesterday than I didn't on my short walk to work today, but I get nothing for it. 

So I guess im saying the tracker is inconsistent and doesn't really work.

[Rich_Laue]

Active minutes are based on calories consumed during the minute, these minutes also must fall within a ten minute period of activity. 

One of the tools at your disposal that might help explain what is happening is to look at the activities log and the Heartrate and Calories graphs