Hi,
It is pretty hard for the program to follow a point that changes shape during the video. This is probably what is happening here.
The foot for exemple rotates around the ankle. The hip may be occluded by the arm at some point, etc.
Also, at any given time, there might be some places in the image that actually looks more like the previous position target than the new position of the target itself. This happens when the point to track is small and background get included in the "target". I have experienced that with knee tracking when the person is far from the camera and the leg looks thin on the image.
I would suggest manual tracking at first. Instead of hitting play, just nudge the mouse scroll up a bit to get to the next frame, and see if the automatic tracking is fine for this very frame.
Do this until the tracking goes nuts, and when it does, re-adjust the position manually.
I understand that doing this over the whole video is fastidious 
One other solution (not always an option) is to place some markers on the person.
If you go that route, try to use uniform circular markers. (basically, their appearance shouldn't change when you rotate or translate them.) I found some cheap circular colorful stickers in the kids department of a general store, they look like this.
You can also set dim light conditions and use reflective / led markers.
A sample video may help understand if the issue is that the tracking scenario is too hard to be automatized in the current version, or if there is indeed a defective behavior that should be addressed.
