Sometimes the source of a roof leaks is difficult to find.
The best approach that I've found for locating the leak is to have someone watch where the water is coming into the house (may be easiest if this can be done in an attic) and have a second person spray the roof with a hose. Spray first at lower elevations and keep spraying the same spot for at least a minute to give the water a chance to make it inside. If the water doesn't show up inside then work your way along the roof at the same elevation. Then work your way from the bottom to the top until water starts coming in. The source of the leak is likely close to the spot you're spraying or where you sprayed last.
What techniques have you found useful?