Unable to Update Multiple Records Using OpenXML

Trying to update multiple records in SQL Server 2008 R2. There are 2 columns to update - AppraisalId & Feedback, after matching the ActivityId. There are no errors, but the update isn't happening. I can see the xml string with data. Where am I going wrong?

Code-behind :

btnSave_Click()
{
  dataset ds = new dataset();
  datatable dt = new datatable;

  dt.columns.add("ActivityId");
  dt.columns.add("AppraisalId");
  dt.columns.add("Feedback");

  foreach(gridviewrow gr in gvmain.rows)
  {
     hiddenfield hd = (hiddenfield)gr.findcontrol("hdactivityid");
     dropdownlist ddl = (dropdownlist)gr.findcontrol("ddlappraisal");
     textbox txt = (textbox)gr.findcontrol("txtfeedback");

     datarow dr = dt.newrow();
     dr["activityid"] = guid.parse(hd.value);
     dr["appraisalid"] = ddl.selectedindex > 0 ? ddl.selectedvalue : null;
     dr["feedback"] = txt.text.trim();

     dt.rows.add(dr);
   }

   ds.tables.add(dt);
   objDAL.SaveAppraisals(ds.GetXml());
}

DAL:

public void SaveAppraisals(string xmldoc)
{
  sqlcommand cmd = new sqlcommand("SaveAppraisals",con);
  cmd.CommandType = CommandType.StoredProcedure;
  cmd.Parameters.AddWithValue("@xmldoc", xmldoc);
  con.open();
  cmd.executenonquery();
  con.close();
}

SQL:

alter procedure SaveAppraisals
   @xmldoc xml
as
begin
 set nocount on;
 declare @doc int 

 exec sp_xml_preparedocument @doc output,@xmldoc 

 update tblActivity 
 set AppraisalId = ox.AppraisalId, 
     Feedback=ox.Feedback
 from openxml(@doc,'NewDataSet/Table1', 2) 
      with(ActivityId uniqueidentifier, AppraisalId int, Feedback) ox 
 where tblActivity.ActivityId = ox.ActivityId 

 exec sp_xml_removedocument @doc
end