Why is my camera not focusing

In the Canon SX200IS which I'm familiar with, the focussing lens is the last element before the sensor, and this moves in and out on a brass insert which engages in a leadscrew. (See my SX200IS Lens Disassembly Guide for a full description.) It's likely that your lens is a similar design, in which case I would suspect that this mechanism is faulty and only works on part of its travel. I saw an example of a heavily used camera (probably many thousands of exposures) where the thread on the brass insert was so worn as to no longer work. This could be your problem. Not having access to watchmaker's tools, the only way I could fix it was to cannibalise the part from another broken camera.