impalpable, unidentifiable, subtle, evasive, artful, searchless, evanescent, unglimpsable, opaque, recondite, more...

obvious, clear, evident, apparent, visible

hard to find, rare, difficult to catch, hard to pin down, more...

hard to find,   hard to get,   hard to grasp,   hard to describe,   hard to pin down,   difficult to find,   hard to catch,   easy to miss,   hard to detect,   not easy to find