I think 未開拓の潜在能力 is fine. Alt. translation of "untapped potential" is "潜在的（な）可能性."
It sounds a bit odd, but the direct translation would be
常に潜在的可能性でいられるとは限らない。(限りません is fine, too.)
Translation would be easier if you could possibly change the line to:
You can't always be a person with untapped potential.
Last edited by mikkih; June 14, 2014 at 07:12 AM. Reason: title edit due to the move: Translation Help (English to Japanese)