Voldemort had not become the master of the Elder Wand, therefore it would not fully obey him. To become the master of a wand, one must defeat the former master. Dumbledore had the master but Draco Malfoy Disarmed him, making him the unknowing master. Later, Harry Potter Disarmed Malfoy, and so he bcame the master of the Elder Wand. Voldemort's first attempt to kill Harry with the Elder Wand failed because Lily Potter's blood was inside Voldemort and so that blood protection kept Harry alive. The second time, the Wand recognized Harry as its master, and so, when Harry Disarmed Voldemort, the Elder Wand rebounded the Killing curse on Voldemort, then flew to meet Harry.