:? are your changing it on 'M' or 'A'?
dont know if that might affect it?
if you put it on manual focus (dial thing on the left side of the camera (c-af, s-af, mf) then if you press the magnify button on the back of the camera so zoom into the centre you should be able to get a better idea of what is in focue then use the focusing ring on the front to get it perfect
ummm other than that not sure
edit: forgot to say the camers does try and help you when using the magnify thing as you get little symbols underneath (arrows to show which way to turn the dial and a dot when it is in focus)