앞에 있는 손잡이 말고 뒤에 있는 조그마한 기어변속 버튼을 누르면 한 번에 기어 변속이 되지 않고 플라스틱이 부러지는 것 같은 소리가 들렸다가 더 누르면 기어변속이 됩니다ㅠㅠ 불량품일까요?