I'm trying to delete the old markers and load new ones.

Here is the code I have that loads certain markers on page load - no issues here:

            (function() {

            var customIcons = {
  1: {
    icon: 'redmarker.png',
    shadow: 'markershadow.png'
  },
  2: {
    icon: 'purplemarker.png',
    shadow: 'markershadow.png'
  },
  3: {
    icon: 'silvermarker.png',
    shadow: 'markershadow.png'
  },
  4: {
    icon: 'goldmarker.png',
    shadow: 'markershadow.png'
  }
};


                window.onload = function(){
                    var MY_MAPTYPE_ID = 'custom';
                    var stylez = [ { "stylers": [ { "hue": "#00ccff" }, { "saturation": -100 }, { "lightness": 5 } ] },{ } ];
                    var latlng = new google.maps.LatLng(10, 10);
                    var options = {
                      zoom: 16,
                      center: latlng,
                      panControl: false,
                      zoomControl: false,
                  scaleControl: true,

                      mapTypeControlOptions: {
                        mapTypeIds: [MY_MAPTYPE_ID,google.maps.MapTypeId.SATELLITE]
                      },

                      mapTypeId: MY_MAPTYPE_ID
                    }; 
                    var map = new google.maps.Map(document.getElementById('map'), options);
                    var styledMapOptions = {
                        name: 'Map'
                    };
                    var jayzMapType = new google.maps.StyledMapType(stylez, styledMapOptions);
                    map.mapTypes.set(MY_MAPTYPE_ID, jayzMapType);


                    var infoWindow = new google.maps.InfoWindow;

  // Change this depending on the name of your PHP file
  downloadUrl("getxml.php", function(data) {
    var xml = data.responseXML;
    var markers = xml.documentElement.getElementsByTagName("marker");
    for (var i = 0; i < markers.length; i++) {
      var name = markers[i].getAttribute("id");
      var address = markers[i].getAttribute("id");
      var type = markers[i].getAttribute("venue_type");
      var point = new google.maps.LatLng(
          parseFloat(markers[i].getAttribute("lat")),
          parseFloat(markers[i].getAttribute("lng")));
      var html = "<b>" + name + "</b> <br/>" + address;
      var icon = customIcons[type] || {};
      var marker = new google.maps.Marker({
        map: map,
        position: point,
        icon: icon.icon,
        shadow: icon.shadow
      });
      bindInfoWindow(marker, map, infoWindow, html);
    }
  });



//BUTTON SWITCHING ////////////////////////////////////////////////////////////
//BUTTON SWITCHING ////////////////////////////////////////////////////////////
//BUTTON SWITCHING ////////////////////////////////////////////////////////////
//BUTTON SWITCHING ////////////////////////////////////////////////////////////
//BUTTON SWITCHING ////////////////////////////////////////////////////////////
//BUTTON SWITCHING ////////////////////////////////////////////////////////////
//BUTTON SWITCHING ////////////////////////////////////////////////////////////
jQuery(document).delegate(".topCanBeActive", "click", function( e ) { 
    e.preventDefault();
    jQuery(".topCanBeActive").removeClass("topActive");
    jQuery(this).addClass("topActive");

    switch( this.id ){
            case 'all_activity_button':
                     alert("search");
          break;
        case 'events_button':

downloadUrl("getxml2.php", function(data) {
        var xml = data.responseXML;
        var markers = xml.documentElement.getElementsByTagName("marker");
        for (var i = 0; i < markers.length; i++) {
          var name = markers[i].getAttribute("id");
          var address = markers[i].getAttribute("id");
          var type = markers[i].getAttribute("venue_type");
          var point = new google.maps.LatLng(
              parseFloat(markers[i].getAttribute("lat")),
              parseFloat(markers[i].getAttribute("lng")));
          var html = "<b>" + name + "</b> <br/>" + address;
          var icon = customIcons[type] || {};
          var marker = new google.maps.Marker({
            map: map,
            position: point,
            icon: icon.icon,
            shadow: icon.shadow
          });
          bindInfoWindow(marker, map, infoWindow, html);
        }
      });


          break;
        case 'venues_button':
            alert("venues");
          break;
        case 'search_button':
            alert("search");          
          break;
    }
});


//END ////////////////////////////////////////////////////////////
//END ////////////////////////////////////////////////////////////
//END ////////////////////////////////////////////////////////////
//END ////////////////////////////////////////////////////////////
//END ////////////////////////////////////////////////////////////
//END ////////////////////////////////////////////////////////////
//END ////////////////////////////////////////////////////////////
//END ////////////////////////////////////////////////////////////


                }

                function bindInfoWindow(marker, map, infoWindow, html) {
  google.maps.event.addListener(marker, 'click', function() {


    infoWindow.setContent(html);
    infoWindow.open(map, marker);
  });
}

function downloadUrl(url, callback) {
  var request = window.ActiveXObject ?
      new ActiveXObject('Microsoft.XMLHTTP') :
      new XMLHttpRequest;

  request.onreadystatechange = function() {
    if (request.readyState == 4) {
      request.onreadystatechange = doNothing;
      callback(request, request.status);
    }
  };

  request.open('GET', url, true);
  request.send(null);
}

function doNothing() {}

            })();

Now, I created a button section where if you press one button, a different xml file is loaded. Notice the section with the ////////////////////// However, upon clicking the button, nothing happens. The xml file itself is okay and loads the desired data. I also receive no errors in firebug.

Any ideas why this happens?

Thanks!