Adding JS to header

I’m trying to get a ‘video pausing’ bit of js to work on my site. Bit odd – when I add it to the ‘header.php’ it doesn’t work, though I can see it on the page. Works fine when I add it inline in a ‘code block’

<script>
jQuery(document).ready(function ($) {
    "use strict";
    
    /* activate pause for lightbulb video if scrolled out of viewport */
    $(window).scroll(function() {
        $('video').each(function(){
            if ($(this).is(":in-viewport( 400 )")) {
                $(this)[0].play();
            } else {
                $(this)[0].pause();
            }
        });
    });
});
</script>

Any ideas why?