My daughter competes in bicycle and ski races so I often take photos with a Canon EOS T2i and an AF 70-300 mm f/4-5.6 zoom lens but many shots are not in sharp focus. When I shoot a series of nine photos of her racing toward me, maybe four will be sharp while the others will not quite be in focus. My brother has a Sigma 70-200mm f/2.8 HSM zoom and a lot more of his action photos are sharp.
Your camera has a very good continuous (predictive) autofocus system that can track a moving subject effectively, Joey. For the best results, be sure to set it to the Servo AF option, not the AIA autofocus mode. The latter does activate continuous autofocus when the system detects motion but may not do so immediately while Servo AF provides full-time tracking focus. If the action is moving very fast (downhill for example) I also suggest activating only the single, central focus detection point. It will allow the camera to provide the fastest possible continuous autofocus, however it will require you to center the primary subject and it works best when the subject is close to the camera.
Granted, even then, some of your action photos may not be sharply focused, especially when made at 200 mm to 300 mm and on overcast days. That’s because your lens’ maximum aperture is quite small at long focal lengths—that minimizes the amount of light reaching the autofocus sensor. By comparison, an f/2.8 lens (with f/2.8 at all focal lengths) provides far greater light transmission for superior AF response. When possible, use the 70-190 mm range with your lens for action photography on dark days because the aperture is still acceptably wide (f/4 or f/4.5) then. Also, be sure to use a fast shutter speed such as 1/1000 s. Often, blurring that’s diagnosed as caused by a focusing problem is actually motion blur caused by a shutter speed that did not fully freeze the motion.
As well, your brother’s lens is equipped with an ultrasonic focus motor (called HSM by Sigma) and internal focusing (IF), which are definite benefits over your zoom. If you’re serious about action photography, consider buying a lens with ultrasonic AF such as the Sigma 70-200 mm f/2.8 HSM or Canon 70-200 mm f/4L USM. (Longer focal lengths are very useful for races so you’d also want the matching 1.4x teleconverter or you might prefer a 300 mm f/4 lens.) I have tested the recommended zooms when shooting go-kart races and equestrian events and can confirm that both provide superb image quality and very fast tracking autofocus.