IE6 and IE7 ExtJS gridPanel Header disappear

I am using ExtJS v 3.3. this is my grid:

    var grid=new Ext.grid.GridPanel({title:'',store: store,
                              trackMouseOver:true,
                              disableSelection:true,
                              loadMask:true,
                              columns:[{id:'srf2', header:'<b>שם צרופה</b>', dataIndex:'fileName', width:'auto', sortable:true},{id:'cbDocType', header:'<b>סוג מסמך</b>', dataIndex:'cbDocType', width:'auto', sortable:true},{id:'cbDocSubject', header:'<b>נושא מסמך</b>', dataIndex:'cbDocSubject', width:'auto', sortable:true},{id:'txtFromDocDate', header:'<b>תאריך מסמך</b>', dataIndex:'txtDocDate', width:'auto', sortable:true},{id:'txtDocName', header:'<b>שם מסמך</b>', dataIndex:'txtDocName', width:'auto', sortable:true},{id:'cbInitiativeDepartment', header:'<b>חטיבה יוזמת</b>', dataIndex:'cbInitiativeDepartment', width:'auto', sortable:true},{id:'cbDepartmentInitiates', header:'<b>אגף开发者_如何学编程 יוזם</b>', dataIndex:'cbDepartmentInitiates', width:'auto', sortable:true},{id:'cbBuyerDepartment', header:'<b>ממ&quot;ח רכש</b>', dataIndex:'cbBuyerDepartment', width:'auto', sortable:true},{id:'cbEconomist', header:'<b>כלכלן</b>', dataIndex:'cbEconomist', width:'auto', sortable:true},{id:'txtDemandNumber', header:'<b>מספר דרישה</b>', dataIndex:'txtDemandNumber', width:'auto', sortable:true},{id:'txtCallNumber', header:'<b>מספר התקשרות</b>', dataIndex:'txtCallNumber', width:'auto', sortable:true},{id:'txtSupplier', header:'<b>ספק</b>', dataIndex:'txtSupplier', width:'auto', sortable:true},{id:'txtFromDocUploadDate', header:'<b>תאריך העלאה</b>', dataIndex:'uploadDate', width:'auto', sortable:true},{id:'userName', header:'<b>שם משתמש</b>', dataIndex:'userName', width:'auto', sortable:true}],
                              stripeRows:true,
                              viewConfig:{forceFit:true,
                              enableRowBody:true,
                              showPreview:true
                              }
,
                              sm:new Ext.grid.RowSelectionModel({singleSelect:true})

                              ,bbar:[new Ext.PagingToolbar({id:'paging-bar'
                              ,pageSize:30
                              ,store:store
                              ,displayInfo:true
                              ,displayMsg:localize.displayPages
                              ,emptyMsg:localize.noDocsToDisplay
                              ,listeners:{beforechange:onBeforePageChange}
})],listeners:{rowcontextmenu:onRowcontextmenu},contextMenu:new Ext.menu.Menu({items:[{pressed:false,
                              enableToggle:false,
                              text:localize.deleteRecord,id:'delete_attachment'}
,{pressed:false,
                              enableToggle:false,
                              text:localize.downloadRecord,id:'download_attachment'}
,{pressed:false,
                              enableToggle:false,
                              text:localize.updateRecord,id:'update_attachment'}
],listeners:{itemclick:onItemclick}}),autoExpandColumn:'userName',id:'searchResultPanel'});


Ext.onReady(function(){
                                                var dynamic_grid = new Ext.Container({
                                                                        layout:'fit',
                                                                        items:grid
                                                                    })
                                                dynamic_grid.render('dynamic_grid');
                                                Ext.getBody().on('contextmenu', Ext.emptyFn, null, {preventDefault: true});
                                                //onSubmit(false);
                                                });

once I refresh the grid either with the PagingToolBar or Update the store records IE6 and IE7 make the grid headers disappear. Any suggestion on why this happens?

After a little digging I found that the problem was the loading mask of the grid. I am still not sure why this is happening but once I have removed the grid masking the headers stayed fixed in there position.