I do get baffled by the tracking thing. I don't get a good track when I expect to and vice versa. I have this HUD animation which follows the actor perfectly via a tracking marker placed on an earlobe. The actor is walking away from the camera and I fully expected the marker to lose its position - but I was pleasantly surprised (the actor ends up a long way from the camera).
However, when I had the same actor standing on the spot and bending his arm at the elbow because he's looking at his missing lower arm that's just been blown off - Mocha Ae had trouble following that simple and slow movement - despite some high contrast going on - a basically white arm against a sea of grass. While playing with the spline tool though, I noticed the more accurate setting up of the track the better (of course). So, I drew around the arm more accurately after the first track failed - but the clincher was adding a ton of handles. I swamped the shot with them and the result was a very accurate track.