Literal:
* Elusive: Implies something that is difficult to catch or obtain.
* Out of grasp: A straightforward way to describe something physically unreachable.
* Within arm's reach: This phrase implies proximity, but also highlights the frustrating fact that something is still unattainable.
Figurative:
* Just beyond reach: Implies a close but frustrating distance.
* Almost attainable: Suggests that something is close to being achieved, but not quite there yet.
* Tangible yet distant: A poetic way to describe something that is both real and present, but also out of reach.
The best word to use will depend on the specific situation and the emotion you want to convey.