A problem with controlling shadows was what to do when there were no shadows. Moonless or cloudy nights were the perfect example of this weakness. With cloudy skies came occasional thunderstorms, and storms brought flashes of lightning. With lightning shadows returned, giving the Nara clan seconds of power.
Shikamaru thought of Temari's eyes as a storm. He never functioned when she looked at him, the clouds of her eyes preventing any shadows he could use to drown his growing feelings.
That cloudy night they were trapped by 10 missing-nin, Shikamaru felt useless. But when Temari's eyes flashed defiance of their opponents, he felt his shadows return, by her eyes' lightning.