Maybe the amount he's moving his head is too much? Trytracking on something like a button on a shirt. Something central that isn't moving about too much.
Hope this helps
even when there is something to track this isn't always easy. There's parallax, focal length, and focus issues as well. Background replacement is a combination of art and science. You might try point tracking on the thing in the frame that has the least amount of movement. You also might try stabilizing the footage so the actor does not move, dropping in the background, adding a null, attaching the null's position property to the stabilized footage Anchor point with an expression, then parenting both the background and the stabilized footage to the null to remove the stabilization from the footage and add the motion to the background. See this example:
A screenshot or little sample of the video would help a lot. There's usually at least something in the frame you can track.
The head doesn't move that much more than the rest of his body, the trouble was mostly that he shakes his head a little bit. That's when the background would go nuts. I can get a perfect track from his head. There were two different points that stuck really well. His nostril and (even better) his ear lobe. I couldn't get anything below his neck (or the neck itself) that I could track. Here's a screenshot of the scene.
I'm going to try the route with the null layer, I had forgotten about that option. As far as the video that you posted goes, is that supposed to have audio with it? That's the only thing that made it really hard to follow. I'm going to keep plugging away to see if I can get this to work. I'll be back either with more questions or to mark this answered. Thanks for the help!
I think you are stating that you have camera movement of a hand held, and the still you have in the BG looks un-naturally still.
This is where you want to apply :
Track motion>Tracktype- Transform>>Add Rotation,Scale Set your Options to (extrapolate Motion) Clean up your KF's after track via delete the ones that are unwanted. Less then 80% or more confidence, edit to suit. Since AE will interpolate the movement.
EDIT> also under option you have choices for saturation or color. That neck line of blue to flesh is a good RGB choice-
If your track moves your BG all Willy Nilly'
Open the BG layer's Anchor Point Scrub to fit to frame.
Now you can use a Null to attach both layers to if you desire a Zoom, pan so both are tied together