Lets start off by defining some terms. After Effects does not have a 3D tracker, it has a camera tracker. Camera Tracking involves looking for detail in fixed geometry in the shot that is at different distances from the camera, then calculating the shift in parallax to recreate a virtual camera that moves in the same way the camera used to shoot the video was moving. Anything that is moving in the shot can cause problems with the calculations. The more things that are moving the more likely it is that you cannot get a solution when the software is solving the camera.
For example, a shot of a bunch of folks swimming in the ocean with nothing but water and swimmers in the shot cannot be solved because everything in the shot is moving. A shot of a huge crowd leaving a football stadium that does not include a lot of the stadium in the shot is going to be very difficult to solve because most of the detail in the shot is moving. There is nothing to measure. A shot following someone walking down the sidewalk on a city street may also be difficult to solve if the camera is perpendicular to the buildings because there is little or no perspective change in the shot because everything except the actor is on the same plane and there may be reflections in the windows that are moving in the wrong direction. A shot with a wide angle lens and a lot of distortion may be difficult or even impossible to solve because the Camera Tracker doesn't know that the edges are distorted.
Without seeing your shot we can only assume that it was not suitable for camera tracking or poorly planned. You need to do some studying on the subject and carefully plan and shoot shots that you intend camera track to insure that you have a successful solution.
I hope this helps. The search help field at the top right corner of AE is a good place to start looking for training on Camera Tracking.
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