elusive

(s) difficult to describe
- a haunting elusive odor
(s) skillful at eluding capture
- "a cabal of conspirators, each more elusive than the archterrorist"- David Kline
(s) 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
subtle,
(s) 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
baffling, knotty, problematic, problematical, tough,

subtle baffling knotty problematic problematical tough

elusion

elusiveness