@gouder
From what I can tell, there are three types of people. The first can see ghosts naturally (Hitomi), the second can see ghosts if they choose to show themselves (Yachiko, Kuon, Kotori, Shiori, young Igarashi), and the third can't see ghosts at all (old Igarashi, Furuya, maybe Orie). Rikka's an exception because she's got Hitomi's eye(s?).
Also, it seems like the older you get, the harder it gets to see the spooks.