The selector .nav-ul .menu-item:hover ul will only target the <ul> inside the .menu-item item being hovered.
You can't traverse up the DOM using CSS as of now.
Add the following script:
$('.menu-item').hover(function(){
$(this).siblings().find('ul').hide();
});
Demo
Update:
If you want to hide the dropdown when the mouse moves away, you can use the second callback of hover, as given below:
$('.menu-item').hover(function () {
$(this).siblings().find('ul').hide();
},
function () {
$(this).find('ul').hide()
});
Demo
Side note: For anyone down voting seeing Yury Tarabanko's comment, it's not a reliable solution for the task at hand (it doesn't work if the submenu is before the hovered item).