Does not show one's potential

Solution 1:

One of the most-used terms for such a person is underachiever, which wiktionary defines as “One who underachieves by performing less well than expected”. But slacker, in its sense of “A person lacking a sense of direction in life; an underachiever” may often apply too, given the “never gives the effort to show [potential]” stipulation in the question.

Solution 2:

The adjective unfulfilled might be used to describe this kind of person.

of persons; marked by failure to realize full potentialities; "unfulfilled and uneasy men"

_{Source: http://www.thefreedictionary.com/unfulfilled}

It is related to not realizing rather than effortlessness, but you might not realize your potential if you do not show any effort. Thus, effortless conveys this idea too.

The opposite of what you are asking is fulfilling your potential. Macmillan dictionary has the following definition and an example with a negated sentence to convey your idea:

fulfill your potential

to achieve as much as your natural ability makes possible

He never really fulfilled his potential as a player.

Solution 3:

If you are giving it a "positive" connotation, instead of negative, perhaps these can get close:

modest, humble, unpretentious, unassuming