Nemohu dostat některá data do formuláře Ajax

{
  "artist": "reziser",
    "release_dated": "22",
  "release_datem": "11",
    "release_dater": "1902"  
}

<input name="artist" type="text" id="must" class='artist' value="" style="width: 500px;" />
<td>Den <input name="release_dated" type="text" size=2 maxlength=2 class='release_dated' />
                Mesic <input name="release_datem" type="text" size=2 maxlength=2  class='release_datem' />
                Rok <input name="release_dater" type="text" size=4 maxlength=4 class='release_dater' /></td>
             
<script type="text/javascript">
    function loadni(data) {
        $(".artist").val(data.artist);
        $(".release_dated").val(data.release_dated);
        $(".release_datem").val(data.release_datem);
        $(".release_dater").val(data.release_dater);
    }

$("#ajax").click(function(){
        $.getJSON('getinfo.php', {'promenna' : $("#xx").val()}, loadni);
});
</script>