Fixed background image not working on iPhone

Hi!

I just launched my first ever wordpress theme, and almost everything is running great. BUT, for some reason my fixed background image is not fixed on my iPhone SE (neither Safari nor Chrome). I’ve tried it on an LG G2 Android 4.4.2 Kitkat and there it works perfectly.. Also on a Samsung S4.. Anyone have an idea what’s wrong?

Website link: http://www.storhamar-kunstlop.no

Here’s an excerpt from functions.php:

$args = array(
	'default-color' => '004E9C',
	'default-image' => get_template_directory_uri() . '/images/bg.jpg',
	'default-attachment' => 'fixed',
	'default-repeat' => 'none',
	'default-position-x' => 'center',
	'default-position-y' => 'center',
	'uploads' => true,
);
add_theme_support( 'custom-background', $args );

And here’s an excerpt from my css:

body {
    background: url('images/bg.jpg') rgb(0,78,156) no-repeat center center fixed;
    font-family: "Source Sans Pro", Helvetica, Arial, sans-serif;
	font-size: 14px;
	color: rgb(0,78,156);
}

Hope someone can help. Thanks!
Elena