Charge 3 won't track calories burned properly

[Dasher13]

My Fitbit Charge 3 does not calculate the calories correctly. 
On some days I will, for eg, walk 5km and the calories will prob be correct. Following day I might walk or run 10km and the calories burnt are less than when I’m doing very little. 
I’ve tried everything. Reinstalling the app, turning my device off and on. The help desk have been very unhelpful and doubly disappointed as this is the 2nd Fitbit I’ve had and exactly same problem. You never know how your Fitbit is going to perform each day!

Moderator edit: updated subject for clarity

[JohnnyRow]

1. Use the Exercise App rather than relying on auto-recognition so you know it is counting the full time of your workout.

2.  Look at the exercise summaries, total calories, calories/minute, heart rate, total time.  Is it really showing lower calories burned/minute and lower heart rate while on your run than while on a walk or "doing very little".  Post screen caps of them here so we can see real data to help make sense of it.

[JohnnyRow]

Sorry, but I don't have long-distance x-ray vision to be able to see your data.

I can't see your activity performance any better than you can see mine.  I'm not a fitbit employee;l we are just fitbit users trying to help each other.

[Dasher13]

Whoops sorry!   I’d emailed Fitbit and thought you were responding as a FB help desk member.