Something that's unavailing doesn't achieve the desired results. After an unavailing search for a pair of matching socks, you were forced to wear a blue and green striped sock on one foot and a pink polka-dotted sock on the other.
When your efforts are ineffective, they're unavailing. A teacher's unavailing attempt to keep your class quiet means that the noise continues, and an underdog basketball team's unavailing struggle to make up the ten-point lag at halftime means they ultimately lose the game. This adjective comes from the now-obsolete availing, "advantageous," combined with the prefix un-, "not."