The reason you can't "see" ghosts sometimes and can other times is because they're trapped between dimensions. There are up to 11 dimensions. Humans can only see up to the 3rd. Although, ghost are typically trapped between the 3rd and 4th. When ghosts are closer to the 3rd is when they manifest and you can see them. The 4th dimension consists of nothing but pure energy and vibrations. Music, which we can not see, exists in the 4th dimension. The reason we can hear it, is because the vibrations of the music is high enough for our ears to hear. Ghosts' vibrations vary, therefore sometimes we can hear/see them, other times, we can not, also we do have photographic evidence for the existence of ghosts.