Newbie question - conflict with imported classes?

An unashamedly naive question, I’m afraid, from someone trying to get to grips with as3 classes…

I have a movie clip (a car) on the stage that’s linked, through the ‘Class’ field in the symbol properties dialogue box, to a class which allows it to move. This works fine, until I attempt to set up collision detection for a movieclip representing the track, by importing a CollisionDetection class; then, the car movement class does not appear to be imported at all. Neither does it appear to work when I include any import statement on the main movie’s timeline.

Could anyone suggest why this might be the case?