I am new to programming and having an issue with my navigation bar. I have a fully functional nav bar and when I scroll down it hides as expected but does not pop back up when I scroll back up. This is the function I used for the scrolling:

var scrolled;
var lastscroll = 0;
var delta = 5;
var navHeight = $('header').outerHeight();

$(window).scroll(function(event){
    scrolled = true;
});
setInterval(function() {
    if (scrolled) {
        hasScrolled();
        scrolled = false;
    }
}, 250);
function hasScrolled() {
    var scrolltop = $(this).scrollTop();
    if(Math.abs(lastscroll - scrolltop) <= delta)
        return;
    if (scrolltop > lastscroll && scrolltop > navHeight){
        $('header').removeClass('nav-down').addClass('nav-up');
    } else {
        if(scrolltop + $(window).height() < $(document).height()) {
            $('header').removeClass('nav-up').addClass('nav-down');
        }
    }
    
    lastScrollTop = scrolltop;
}

Does anyone know what I am missing?