Upgrade from MSP 15 to MSP 17, so Track Motion is new for me.

1) Tried motion tracking on video clip of a person standing & moving in a dimly lit room. Tracking key frame control ran for five to fifteen seconds, then quit. Could not complete tracking the clip. Tried again on a clip with better lighting and this works, but:

2) Second clip is sidewalk vendor. Put tracking mask on pedestrian #1. Mask tracks person 1 to vendor. As person 1 passes vendor, the mask stops on vendor!  Second pedestrian #2 passes vendor and mask now follows person 2 to end of clip! Any ideas on how to avoid this??