The slow zoom is one of the most difficult things to actually get right when filming. Even if you are controlling the scene, doing the zoom consistently and on queue is next to impossible no matter what is going on. It can start and stop, change speeds, or come in late. Instead of doing this you can use the principles of photo motion to do this in the editing room. Just take the video clip and set the in and out points in the motion tab where you would like to put the zoom in. Then put the change in size that you want on the end point, and even adjust the image somewhat. This will make it appear to zoom in to the live video. It is best if you do it only a slight bit over an extended period of time, unless you are trying to achieve a novelty effect with a fast zoom. It is also important that it was filmed with a relatively high resolution camera otherwise you may end up out of focus at the end.