Rebel xti lens . strange problem for me.?

Did you go into the options menu for your camera to see if your focal options were correct? When you look into your viewfinder, and you focus, does it ony focus on one area? (you should have a little square or circle flashing that tells you what you are focused on)You can usually change this to focus on more than one thing. Your camera should have a manual that explains this. I would figure that out before buying another lens.