Motion's tracker is a point tracker. You really need a planar tracker to do what you want. Sometimes you can get it to work - make sure you are doing a Corner Pin track - but really, much better to use a plugin like TrackX which works in both Motion and FCP X.
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press http://www.applemotion.net