为何无法翻页

以下代码可以获得数据，但是无法通过点击翻页来获得翻页效果，两种proxy都可以获得数据，但无法得到翻页的参数start和limit，请问该如何解决？环境：xp+domino R7
    var store = new Ext.data.Store({

// proxy: new Ext.data.HttpProxy({url:'agjson?openagent'}),
proxy: new Ext.data.HttpProxy({url:'vwjson?openview'}),

reader: new Ext.data.JsonReader({
root:'topics',
totalProperty:'totalCount',
id:'id'
},[
{name: 'id'},
{name: 'title'},
{name: 'author'}
        ])
    });

    var grid = new Ext.grid.GridPanel({
        store: store,
        columns: [
            {header: "id", width: 160, sortable: true, dataIndex: 'id',hidden:true},
            {header: "title", width: 700, sortable: true, dataIndex: 'title'},
            {header: "author", width: 400, sortable: true, dataIndex: 'author'}
        ],
        height:350,
        width:700,
        title:'权限管理',
        trackMouseOver:false,
        sm: new Ext.grid.RowSelectionModel(),
        loadMask: true,
        viewConfig: {
            forceFit:true,
            enableRowBody:true,
            showPreview:true
        },
        bbar: new Ext.PagingToolbar({
            pageSize: 15,
            store: store,
            displayInfo: true,
            displayMsg: 'Displaying topics {0} - {1} of {2}',
            emptyMsg: "No topics to display"
        })
    });

    grid.render('grid-example');
    store.load({params:{start:0, count:12}});

    grid.getSelectionModel().selectFirstRow();
    grid.on("rowdblClick",function(grid,rowIndex,e){
alert(store.getAt(rowIndex).data.title);
    });