I see this error in the console:
Uncaught ReferenceError: jQuery is not defined at (index):3056
which is related to this script in your footer:
<!-- WooCommerce JavaScript -->
<script type="text/javascript">
jQuery(function($) {
$( '.add_to_cart_button:not(.product_type_variable, .product_type_grouped)' ).on( 'click', function() {
gtag( 'event', 'add_to_cart', { 'event_category': 'ecommerce', 'event_label': ($(this).data('product_sku')) ? ($(this).data('product_sku')) : ('#' + $(this).data('product_id')), 'items': [ {'id': ($(this).data('product_sku')) ? ($(this).data('product_sku')) : ('#' + $(this).data('product_id')),'quantity': $(this).data('quantity')} ] } );
});
});
</script>
It could be also breaking the sticky nav, as one JS error can cause other scripts to stop functioning.