问jquery动画是第一次触发，但不是在更换类之后触发？

EN

function slider_minus(){

    $('.left_slot').animate({
        left: '50px'        //only works on first function call
    },300);

    $('.middle_slot').animate({
        left: '50px'        //only works on first function call
    },300, function(){
         $('.middle_slot').attr('class','right_slot')
         $('.left_slot').attr('class','middle_slot')
    });

    $('.right_slot').animate({
        opacity: 0,         //works correctly every time
        left: '50px'        //only works on first function call
    },300);
}