Elusive (adjective)

1. Difficult to find, catch, or achieve.
2. Hard to pin down or remember.
3. Hard to grasp or define.

Origin:

Late 16th century: from latin elus- "evaded", from the verb eludere.

Examples:

1. The answer to the problem was elusive.
2. He had an elusive quality that made it hard to get close to him.
3. Her memories of the event were elusive.
4. The truth was elusive, as many conflicting stories were told.
5. Success has been elusive for the company despite their best efforts.