background image do not display on iphone 5

Question

when i test website my chrome desktop and mobile emulatation, it still work

But when i check on iphone 5, banner background is not display

this my css

.banner .backImg {
  background-attachment: fixed;
  background-position: center;
  background-repeat: no-repeat;
  -o-background-size: cover;
  -moz-background-size: cover;
  -webkit-background-size: cover;
  background-size: cover;
  float: left;
  margin: 0;
  min-height: 772px;
  padding: 0;
  width: 100%;
}