Animate() only works in the first transition

Question

I am creating an infinite Carousel that should be excited between transitions, however the event .animate() only works in the first transition, the rest is completely static, without animation.

Code:

var next = function(){
    var left = parseInt(ul.css('left')) - (size + rest + settings.margin);

    ul.stop(true, true).animate({'left' : left}, settings.speed, function(){
        $('> li:last', ul).after($('> li:first', ul));
        ul.css({'left' : (size + rest + settings.margin) * (-1)});
    });

    return false;
}

var transition = null;

var init = function(){
    transition = setInterval(function(){
        next();
    }, settings.delay)
};

init();

Note: if I remove the fix .css() from the position, Carousel stays excited, but logically ul will be going left until it disappears.

What’s wrong with it?