Motion tracker losing object.

Motion tracking often requires you to manually adjust the tracker position when the object (your player) becomes obscured, moves too fast, or has insufficient detail to be successfully tracked. 

You might consider tracking in Mocha AE. I seldom use AE's tracker unless the shot is very simple.

The outer tracking box is the area AE looks in to identify the shapes in the inner box.  I suspect the ball is leaving the tracking area sonAE then tries to match the grass.  If you make the outer box larger, the ball should continue to track.

In the tracking panel itself if you click on the options button you'll find different ways AE can compare images RGB, luminance etc... which might also give you a better track.

Check your settings for feature adaptation and so on an. That aside your shot may simply not be particularly suitable for tracking. Based on your images there are a lot of large compression artifacts and it's only natural that they would interfere. You may simply have to spend a lot more time and reposition the tracker every few frames then piece things together. Not everything works with a single automated track.

Mylenium