Bit of info on how auto-focus works in most DSLRs. I’ve got a balky 75mm-300mm zoom with auto-focus issues forcing me to “re-discover” manual focus (and not enjoying it as much now that I’m old and half blind), I doubt this will help with that lens’s problems, but it’s worth a shot.
These systems rely heavily on contrast. That’s why when you have low light, low contrast scenes, autofocus rarely works well.
If you want to get the best autofocus possible on your digital camera, aim at an area of high contrast on your subject. For instance, if you are photographing a blonde person against a black background, point the autofocus sensor at the area where the hair and the background merge to get a focus point. Then allow enough depth of field with the proper aperture selection to get the face features in focus.