Grid Panel Scrollbars in Extjs 4 not working

var gusersPanel = Ext.create('Ext.grid.Panel', {
    flex:1,
    columns: [{
        header: 'User Login',
        dataIndex: 'user_login',
        width:150
    },{
        header: 'User Name',
        dataIndex: 'user_nicename',
        width:150
    },{
        header:'Privledge',
        dataIndex:'admin',
        width:150
    }],
    autoScroll: true,
    layout:'fit',
    selModel: gusersSel,
    store: gusersStore

})

Hi I am using above code for the grid Panel in Extjs4.0.2a When I populate data dynamically in the store the scrollbars are not working . I have also tried using doLayout() for grid Panel but dosent work too . The grid Panel is in a Window .

Anything that can solve this problem ?

Actually it works for some time but dosen't work all the time .