So called "sensitives", who can detect ghosts, are not much use in psychical research. The reason is not that it is unlikely (if you are prepared to believe in ghosts), but simply that it is normally impossible to confirm their evidence. After all, if everybody in the room sees a particular apparition, there is a rebuttable assumption that it has an objective existence. However, if only one person claims to have seen it, it may indicate that he is "psychic", but it may also mean that he is (a) lying, (b) self-deluded, or (c) crazy. The situation becomes even more nebulous when it concerns, not seeing the ghost, but merely "sensing" it.
Just the same, there do appear to have been good cases where an apparition was seen by one person, but not another. (Here, for example.) I have recorded a couple in my posts of May 2012 and February 2012. In the latter case, you may recall, a visitor saw the apparition while the owner saw nothing, but the alleged psychics were more or less ineffective. If ghosts are psychic manifestations, then it may well be that one person, because of his or her frame of mind at the time, may be more attuned to it than others (particularly when they are children), and perhaps some people are permanently in that condition.
