Camera’s have come a long way with AF, however, camera AF still requires good knowledge of how the AF system in your camera works, what its limits are, and how to work around those limits when required to do so.
#1 Set your shutter to only release when the image is in focus
On most cameras it may be referred to as focus/shutter priority release. Essentially with this setting you’re telling the camera not to allow the shutter to release until focus has been confirmed on the subject. This means that you can press shutter while the camera is focusing but it won’t actually take the picture until it confirms focus.
#2 Use a smaller aperture
It’s the old school methodology but it is particularly helpful if you’re using a wide angle lens or manual focusing. Stopping down really far to f/8 – f/16 increases depth of field which makes small focus errors less of an issue. See my post on Hyperfocal Distance for more help with this method.
#3 Try manual focus
It may seem like a dumb solution since it isn’t really AF, but sometimes a little twist of the manual focus ring is all it takes to get the shot. If your camera is just having an issue, whether because of the amount of light, the angle, or what have you, stop trying to make it do something it can’t do and just use manual focus in that situation. At the very least you might get one out of a few dozen shots in focus which is better than no shots in focus.
#4 Use a smaller focus box
One way to “help” the camera get focus is to simply use a different focus method. Usually one of the point focus options will work in those situations where tracking may have trouble. Yes, it’s a slight pain to deal with but by using the point method you’re using your own brain to track the subject and then the camera is doing the work of making sure whatever you point the camera at is in focus. This is essentially how most DSLR’s work and typically DSLR focusing systems are a little bit better at this in difficult situations than mirrorless cameras are. High end DSLR PDAF systems are more sensitive at their individual focus points. This has to do with the nature of phase detect AF on DSLR’s. Even with far less light to work with, true phase detect systems are still more sensitive and accurate than mirrorless AF methods. The problem for DSLR systems is focusing errors can be induced when parts of the system are slightly out of alignment.
#5 Try a different point of view
Tracking AF tends to have trouble with busy backgrounds that are relatively too close to the subject. One thing you can try is simply finding a better angle. The more distance there is between the subject and the background, the more the subject will stand out from the background which should help the AF algorithm find the subject and stay focused on it.
While there are technical reasons why AF doesn’t work at times, these techniques should allow you to increase the odds of getting a good quality shot with the camera that you have.