Elusive

adjective
making great mental demands; hard to comprehend or solve or believe
“a baffling problem”
Similar: baffling, knotty, problematic, problematical, tough
adjective
difficult to detect or grasp by the mind or analyze
“his whole attitude had undergone a subtle change”
Similar: subtle
adjective
skillful at eluding capture; - David Kline
“a cabal of conspirators, each more elusive than the archterrorist”
adjective
difficult to describe
“a haunting elusive odor”