Tracking fading object

Not sure what you're expecting there. What's not there is just not there. Eventually the contrast will simply be too low for the tracker to detect any pattern. You may be able to squeeze a bit more use out of it by processing the footage with effects, but eventually you'll just have to manualyl add the last few keyframes based on you gut feeling.

Mylenium

Which tracker are you using? The point tracker, the mask tracker, the face tracker, Mocha AE, the camera tracker, or a third party plugin like Lockdown, King Pin Tracker, or Spot Clone Tracker? There are some tips, but they vary based on the tracker you're using.

You may have to manually track it after a certain point, but doing it every x frames instead of every frame is probably a better plan.