By coincidence, I just used Active Track yesterday morning to follow me on the long driveway leading to my house from the main road after a recent light but pretty snowfall. I wanted to see how the Mavic would perform on "autopilot" in a not-excessively wide environment (i.e., my winding driveway through the forest) that is in a tree-lined area with branches projecting from both sides and in the snow (which DJI instructions say to avoid in Active Track) and to also check out the sensitivity of the visual Over Exposure Warning system.
Personally, I was surprised at how well the Mavic did follow me, maintained its consistent distance from me. and how well it followed me even though there are several right-angle turns on my driveway that the aircraft had to negotiate around to keep me in frame. I had no complaints. Since I believe manufacturing standards are considerably high with DJI products, I would guess that your Mavic and software are fine and your situation may be one of just having a learning curve to deal with in this case.
A couple of things to consider while in this mode is to have a very good fix on your subject (using the green box that appears onscreen) first before you hit GO to engage the Mavic's Active Track software. Also, the ambient light is critical when using this mode. The craft may more easily lose sight of the subject or it will even stop and hover if the lighting conditions aren't adequate. It's not a perfect system, but it does do what DJI claims. Just make sure you have all of your sensors on, too, before flying in this mode so you get the object avoidance protection you need, and ensure you disengage the ability for the Mavic to fly backwards if it senses an object in front of it if you are in a confined space, otherwise your drone can easily end up backing up into a tree, a structure or even a person as there are no rear-facing sensors on board.
I'm not the expert here but that is what I know about it. Hopefully others will chime in with better advice than I can give you. Best of luck getting it to work right and safe flying.