Usually when we reject images for focus, it is because of one of three main problems:
What and how much should be in focus depends on the kind of image. In a straight-on portrait, it's important to have clear focus on the eyes. In a abstracted action shot of a bicycle race, you have a lot more leeway. Stationary objects and macro still-life images are different from fashion shots, all of which are very different from fast-paced sports images. Any kind of texture, like an image of a brick wall that will be used as a background or a full frame worth of coffee beans, needs to be absolutely tack-sharp with sufficient depth of field from corner to corner. You can do it all manual or leave it to your camera. What's important is to identify the important elements and make good decisions about how to treat them.
Image size is a factor in all of this – bigger is better, or at least, more forgiving. A big 24 MP image from a modern digital back gives you more tolerance as far as slight softness goes. If that image is downsized to 3 MP, minor focus issues won't be visible at all. However, we are only talking about minor problems here. If you have completely missed the focus, then the size doesn't matter.
Exposure times are too slow for the given focal length, especially given that the subjects are moving.
Examples of unfortunate focus lock. In the first example the photographer has used the wrong kind of AI camera focusing. In the second a manually focused tilt and shift lens was incorrectly used. Specialty lens usually require a lot of practice for good results.
Full-frame background-type shots with front or top-view perspectives need to be in focus with enough depth of field from corner to corner. This one is too soft at 100%.
Please read this article for more information and examples about our focus expectations.
4.0 Quality Standards
6.0 Title, Description & Keywords
8.0 Model & Property Releases