data changes in grid

i have a datagrid which has checkboxes in it. and a save button
the behavior of my form is that it saves all the changes in the datagrid.
as of now, my form only accepts one changes from the datagrid. eventhough i checked all the checkboxes in the datagrid, it wont save the changes... below is my code:

private void frmApprovingCustomerPO_Load(object sender, EventArgs e)
        {
            BindData();  
        }

        private void BindData()
        {
            CustomerPOApprovalBL custBL = new CustomerPOApprovalBL();
            ds = custBL.PopulateGrid(chkboxApprovedPO.Checked);
            if (this.ds != null)
            {
                this.dataGridView1.AutoGenerateColumns = false;
                dataGridView1.DataSource = ds.Tables[0];
                this.dataGridView1.Columns[0].DataPropertyName = "Purchase No";
                this.dataGridView1.Columns[1].DataPropertyName = "CustomerCode";
                this.dataGridView1.Columns[2].DataPropertyName = "Customer Name";
                this.dataGridView1.Columns[3].DataPropertyName = "PO Date";
                this.dataGridView1.Columns[4].DataPropertyName = "Status";
                this.dataGridView1.Columns[5].DataPropertyName = "Date";
                this.dataGridView1.Columns[6].DataPropertyName = "Approved";
                ds.Tables[0].RowChanged += new DataRowChangeEventHandler(frmApprovingCustomerPO_RowChanged);
            }           
            this.dataGridView1.Refresh();
        }

        private void btnQuery_Click(object sender, EventArgs e)
        {
            BindData();
        }

        private void btnSave_Click(object sender, EventArgs e)
        {
            int retVal;
            CustomerPOApprovalBL custBL = new CustomerPOApprovalBL();

            if (ds != null)
            {
                foreach (DataRow row in this.ds.Tables[0].Rows)
                {
                    if (row.RowState == DataRowState.Modified)
                    {
                        retVal = custBL.updateStatus(ds.Tables[0].GetChanges());
                    }
                }

            }
               
            BindData();
        }

        private void btnCancel_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void frmApprovingCustomerPO_RowChanged(object sender, DataRowChangeEventArgs e)
        {
            string s = e.Row.RowState.ToString();
            if (bool.Parse(e.Row[7].ToString()))
            {
                if (Convert.ToChar(e.Row[5]) == 'A') return;

                e.Row[5] = 'A';
            }
            else
            {
                if (Convert.ToChar(e.Row[5]) == 'U') return;
                e.Row[5] = 'U';
            }
        }

thanks.