elusive
adj.adj. difficult to find, catch, or achieve. You use this for things that seem to disappear just as you are about to get them.
adj. difficult to find, capture, or achieve; tending to evade grasp or pursuit. Often describes abstract concepts like success or physical subjects that are hard to track.
The perfect solution remains elusive despite our hard work.
The rare snow leopard is an elusive creature that few photographers have ever seen in the wild.
While the basic mechanics of the theory were clear, a deeper understanding of its implications remained elusive for even the most senior researchers in the department.
From Latin elusus, past participle of eludo (“to parry a blow, to deceive”).
Often used with verbs like 'remain', 'become', or 'prove'.