51-3d is the one mode where the camera will show you which focus point it is actually using. It works quite well when the subject is easily distinguished from the background, normally by color.
In the other dynamic modes (9, 21, 51) the camera is limited to the number of points you select but only shows the point in the center of the group as this this the one used to establish the focus lock.
The fewer points used, the fewer calculations and less decision making needs to be done by the camera.
Objects moving directly at or directly away from the camera are the hardest ones for the camera to track.
---------+---------+---------+---------+---------+ Joseph K Seattle, WA, USA