如何修复移动版没有覆盖整个页面的bg图片？

body {
  background-image: url('bg.jpg');
  background-size: cover;
}

body {
  background-image: url('bg.jpg');
  background-repeat: no-repeat;
}

body {
  background-image: url('bg.jpg');
  background-position: center center;
}

@media (max-width: 768px) {
  body {
    background-image: url('bg-mobile.jpg');
    background-size: cover;
    background-repeat: no-repeat;
    background-position: center center;
  }
}

@media (min-width: 769px) {
  body {
    background-image: url('bg-desktop.jpg');
    background-size: cover;
    background-repeat: no-repeat;
    background-position: center center;
  }
}