Another Newbie Missing The Obvious?

I am trying to track the door of a car moving across the screen. Moving FASTER than that car and as a result, briefly obscuring it from the viewer, is another car moving across the screen “closer” to the viewer.
The spline on the door of my tracked car gets pushed off by the other car. Am I doing something wrong?
Thank you - any help would be great!

Hi Jim,
Mary has answered your question here: