Can't add the new marker on Google map

Question

In my project each time it run will get all of tweet information from database and point it in the map

but when i use jquery to check if it has the new tweet in database then will get it and point into the map but it's not working

function autoUpdate(){
  var url= "CheckUpdate/";
  $.get(url, Update,

 function(jsonS) {
  $.each(jsonS, function(t,tweet) {
        lat = tweet.Latitude;
        lng = tweet.Longitude;
        point = new GLatLng(lat, lng);
        label = tweet.Text;
        icontype = tweet.Level;    
        map.addOverlay(createMarker(point, label,icontype));   });


   });

setTimeout(autoUpdate,60000);
}

in my createmarker() is

 function createMarker(point, label,icontype) {   return marker;}

How can i add marker each time my autoUpdate function working, Thank you