elusive

difficult to describe; "a haunting elusive odor"

skillful at eluding capture; "a cabal of conspirators, each more elusive than the archterrorist"- David Kline

difficult to detect or grasp by the mind or analyze; "his whole attitude had undergone a subtle change"; "a subtle difference"; "that elusive thing the soul"

making great mental demands; hard to comprehend or solve or believe; "a baffling problem"; "I faced the knotty problem of what to have for breakfast"; "a problematic situation at home"