Fitbit not marking active minutes

[Cookwo]

Three weeks ago I walked 1 mile that automatically counted my walk as a workout. Today I walked 3.5 miles and it didn’t not count as a workout. Same pace, and I don’t stop either time. What will make it identify a workout? 

[JohnnyRow]

First, active minutes and exercise sessions are 2 different things, though they often coincide, and are often confused.

Active minutes are awarded for reaching and maintaining a certain intensity leve for at least 10 consecutive minutes, as described in this help article:

What are active minutes?

Exercise sessions, to count in your weekly exercise days, can be earned 2 ways:

1.  Auto-detected exercises, where Fitbit recognizes you activity and you continue it for whatever time length you have set as the minimum for that activity.

2.  Any activity that you start and stop manually from the tracker.  This is described in this help article:
How do I track my workouts with my Fitbit device?

I prefer method 2.  It guarantees that your workout gets counted as an exercise session, you always the credit for the full time, and you get to identify the activity yourself rather than hope that Fitbit correctly recognizes what you are doing.