What focusing mode do you have it in?
Are you using all points active?
Are you having front/back focus problems?
Have you checked the Zoombrowser EX software, that came with the camera, to find out if the subject that is in focus is the selected focus point?
Is this camera new to you?
Do you have any problems with any other lenses?
Are you locking the focus and then recomposing?
If you don't know how to check the focus point selected then all you have to do is open EOS Viewer Utility. Double click on a thumbnail to enlarge image display. Along the top are icons. The "Show or Hide AF Point" icon is immediately to the right of the IPICT icon (has a little red retangle on it). Click on the "Show or Hide AF Point" icon and it will display the AF point used.
I have a Canon 40D body and I'm using 18-55 IS LENS and there seems to be a problem with the auto focusing problem. How can I fix that? And how much will it cost me to fix it?