elusive -
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"
impalpable imperceptible to the senses or the mind; "an impalpable cloud"; "impalpable shadows"; "impalpable distinctions"; "as impalpable as a dream"
elusive -
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"
difficult,
hard not easy; requiring great physical or mental effort to accomplish or comprehend or endure; "a difficult task"; "nesting places on the cliffs are difficult of access"; "difficult times"; "why is it so hard for you to keep a secret?"