Animate () only works on the first transition

I'm creating an infinite carousel that should be animated between transitions, but the .animate() event only works on the first transition, the rest is completely static, with no 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, the carousel stays animated, but logically the ul will be going left until it disappears.

What's wrong?