Why doesn't work the animate() method with a css top property

Why doesn't work the animate() method with a css top property

Hello I'm trying to move a div with the animate(); Method but doesn't work with the top css property
this is the method that I use

$('.btnControls').click(function(){
        $(".itemContainer").animate({top:"200px"});
});

which is the reason for doesn't work.?

Help me...
Thanks