Shutter speed is only half the equation. What's your aperture set to? If you shoot wife-open (lowest f number) on a fast lens (e.g. f/1.4), the area that is in focus will be too shallow, and the camera's AF will have trouble locking onto fast moving subjects. Step it down to f/4-f/8 to get more of the area in focus,and get the shutter even faster to 1/4000 or even 1/8000 to freeze the motion. Of course if you're shooting indoors that would require cranking up the ISO to compensate, which may introduce noise. It's a matter of finding the right balance.