difficult to find, define or achieve (Oxford)
difficult to describe, find, achieve, or remember (Cambridge)
Eric, as elusive as ever, was nowhere to be found. (Oxford)
Solution to the problem of toxic waste is proving elusive. (Oxford)
Success, however, remained elusive for her. (Cambridge)
elusive , nowhere to be found
Solution is elusive
remained elusive
Ambiguous
Puzzling
Definite
Stable
Elusively
Elusiveness