Please explain focus tracking on D800

Hi everyone, I’m seeking to get a better understanding of how focus tracking works on the D800. What I noticed is that if I set the camera to AF-C with a 9 or 21 (or even 51) dynamic area, select a focus point and press the shutter halfway the camera acquires focus. No problem so far, however if the subject (my kid’s head in this case) moves to the right or left of the focus point, I find that the camera will continue to focus on the same selected point and doesn’t seem to “track” my subject even if they’re still within the other dynamic area focus points (and have not left the frame). When I try to pan the camera to follow my subject as to keep the focus point on them, it’s a hit and miss.

I thought in AFC the camera starts by focusing on the selected point, but may move the focus point to a nearby one if the subject moves… This doesn’t seem to happen in my case. I have tried this with AF-ON and half shutter release, and with setting A3: AF lock-on set to 3 and off. I tried with several lenses (Nikon 24-70 & 70-200) and I seem to get the same results.
I’m pretty certain this is my bad, what am I doing wrong?
Thanks in advance for your help!

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@AT