AUTO FOCUS CAMERA WONT LOCK ON IN AUTOFOCUS MODE ON MOVING SUBJECTS

You did not say which camera body you are using, but you probably have three different auto-focus modes on you camera. You might be using the wrong one.

The modes are as follows:
AF-A Mode: Camera automatically selects single-servo autofocus when AF-A subject is stationary, continuous-servo autofocus when subject is moving. Shutter can only be released if camera is able to focus. AF-S Mode: For stationary subjects. Focus locks when shutter-release button AF-S is pressed halfway. Shutter can only be released when in-focus indicator is displayed.
AF-C Mode: For moving subjects. Camera focuses continuously while AF-C shutter-release button is pressed halfway. Photographs can be taken even when in-focus indicator is not displayed.

You can change this in the menu while in shooting mode. This is from page 48 of the manual:

Choose how the camera focuses.

A (auto) M d (Shooting menu) M I Autofocus mode

A Single AF (default setting)

The camera focuses when the shutter-release button is pressed halfway.

B Full-time AF

The camera focuses continuously until the shutter-release button is pressed down

halfway. Use with moving subjects. The sound of lens drive movement will be heard

while the camera focuses.

All autofocus points, and Focus mode set to Servo.
Check out digitalphotographyschool.com's forums, they are excellent sources for this type of info. And you can join a community of other DSLR users like yourself.

1 ensure that your camera is not set in Macro mode test by taking a picture of a subject very close up ie. 10 - 15 cm if the image is in focus then you may be set in macro mode.
2 if lens is removeable ensure it is fitted properly. remove the lens and refit ensureing the connectors are clean.
3 ensure the switch on the lens or camera is set to AF for auto focus.
4 suspect autofocus sensor is defective if you try to focus on subjects which are close and far away and you do not hear to camera re-setting the focus then it is in manual focus or the focus sensor may be defective or in need of adjustment.

5 recommend specialist have a look at this problem.

usandy,
You can set the focal point by toggling the menu switch (you can see it moving around into different locations as it "wraps" around the viewfinder, now depending on your f-stop the area that is not in focus will be soft, you can also experiment with the autofocus lock on you subject and evn try amnually focusing on your subjects.
randy320sgi

It is just a possibility but you may have overrode the auto focus. If you accidentally move the focussing ring it disables the auto focus.

Chances are good that it's not your camera, but your lens. In a darker atmosphere, you need a fast lens, meaning you need a lens that has a wide aperture (1.4 to 2.8) I'm guessing you're using a lens thats' 3.5 or larger. (Yes, the larger apertures have the smaller numbers)

If your camera was spending all it's time moving in and out trying to focus, it's probably because there was not enough light for the camera to distinguish a focal point. Same thing happens when you shoot a solid blue sky. No focal point.

Help me understand by telling me what lens you're using and what settings you were using on the camera.

The camera uses a precise auto focus mechanism, but under the conditions and with the subjects described below the auto focus function may not work well. Subjects moving at high speed Very shiny subjects such as a mirror or car body Extremely low contrast subjects (such as subjects dressed in the same color as the background, etc.) When there are objects in front of or behind the subject (such as an animal in a cage or a person in front of a tree) Subjects with little reflection, such as hair or fur Subjects with no solidity, such as smoke or flames Subjects viewed through glass In addition, the focus is set on the center of the frame, so if the subject is not at the center (when shooting two people standing side by side, for example), the focus is adjusted on the background and the desired subject (the two people) may be out of focus. In such cases, do the following: Point the camera so that one of the persons is at the center of the viewfinder. Half-press the shutter button. (The focus is locked on the person.) Holding the shutter button in the half-pressed position, reposition the camera to achieve the desired composition. Take the photo. If the focus cannot be adjusted, it is locked to infinity (1.5 meters when using the flash).

1. Typical problem subjects for autofocus 1) Very low-contrast subjects 2) Overlapping nearby and distant objects 3) Very bright subjects in the center 4) Subjects moving very fast 5) Subjects through glass Focus on an object that is at the same distance as the desired subject, apply Focus Lock, and then recompose the picture. Or set the lens focus mode switch to (or), and focus manually. (Manual focus is only possible with cameras providing this feature.) 2. Attempting to take pictures out of the camera's shooting distance: When taking pictures out of the camera's shooting distance, the subject will be out of focus. The shooting distance differs from each camera model. Please check the specifications of your camera in the instruction manual to determine the shooting distance.

1. Typical problem subjects for autofocus 1) Very low-contrast subjects 2) Overlapping nearby and distant objects 3) Very bright subjects in the center 4) Subjects moving very fast 5) Subjects through glass Focus on an object that is at the same distance as the desired subject, apply Focus Lock, and then recompose the picture. Or set the lens focus mode switch to (or), and focus manually. (Manual focus is only possible with cameras providing this feature.) 2. Attempting to take pictures out of the camera's shooting distance: When taking pictures out of the camera's shooting distance, the subject will be out of focus. The shooting distance differs from each camera model. Please check the specifications of your camera in the instruction manual to determine the shooting distance.