home in on (something)
To focus or concentrate on something. It took some time for him to home in on the reason he'd called the meeting.
home in on
Move toward or focus on a goal, as in He began with a couple of jokes before homing in on the main subject of his talk. This expression originally alluded to a vessel, aircraft or missile being guided to its target by a radio beam or some other means. [c. 1920]