i've got one problem when i sending parameter to cr page

the problem is Missing parameter values

in the first time, when my report loaded
it can show parameters

but when i click on export/print/next page
error will come

i'm already test this
my parameter is keeping
but my page told Missing

here is my code

string fDate;
    string tDate;
    string viewBy;
    string fullName;
    ReportDocument report = new ReportDocument();
    ParameterFields pfields = new ParameterFields();

    protected void Page_Load(object sender, EventArgs e)
    {
        crvBinding();
    }
    private void crvBinding()
    {
        fDate = Session["FDATE"].ToString();
        tDate = Session["TDATE"].ToString();
        viewBy = Session["VIEWBY"].ToString();

        string staffID = "0000000001";              //Integrate remove this line
        Session["staffID"] = staffID;               //Integrate remove this line
        Session["StaffID"] = Session["staffID"];

        string strConn = "Data Source=MIXAR-PC;Initial Catalog=StockProductInfo;Integrated Security=True";

        SqlConnection Conn = new SqlConnection(strConn);
        if (Conn.State == ConnectionState.Open)
        {
            Conn.Close();
        }
        Conn.ConnectionString = strConn;
        Conn.Open();

        string sqlStaffName = "select Staff_FName, Staff_LName ";
        sqlStaffName += "from masStaff ";
        sqlStaffName += "where Staff_ID = " + Session["StaffID"].ToString();

        SqlDataAdapter daStaff = new SqlDataAdapter(sqlStaffName, Conn);
        DataSet dsStaff = new DataSet();
        daStaff.Fill(dsStaff, "Staff");

        string fName = dsStaff.Tables["Staff"].Rows[0]["Staff_FName"].ToString();
        string lName = dsStaff.Tables["Staff"].Rows[0]["Staff_LName"].ToString();
        fullName = fName + " " + lName;

        DateTime fd = DateTime.Parse(fDate);
        DateTime td = DateTime.Parse(tDate);
        string FromDate = fd.ToString("yyyy-MM-dd");
        string ToDate = td.ToString("yyyy-MM-dd");

        ParameterField pfieldFDATE = new ParameterField();
        ParameterDiscreteValue disValFDATE = new ParameterDiscreteValue();
        disValFDATE.Value = DateTime.Parse(fDate);
        pfieldFDATE.Name = "FDate";
        pfieldFDATE.DefaultValues.Add(disValFDATE);
        pfieldFDATE.CurrentValues.Add(disValFDATE);

        ParameterField pfieldTDATE = new ParameterField();
        ParameterDiscreteValue disValTDATE = new ParameterDiscreteValue();
        disValTDATE.Value = DateTime.Parse(tDate);
        pfieldTDATE.Name = "TDate";
        pfieldTDATE.DefaultValues.Add(disValTDATE);
        pfieldTDATE.CurrentValues.Add(disValTDATE);

        ParameterField pfieldSTAFF = new ParameterField();
        ParameterDiscreteValue disValSTAFF = new ParameterDiscreteValue();
        disValSTAFF.Value = fullName;
        pfieldSTAFF.Name = "StaffName";
        pfieldSTAFF.DefaultValues.Add(disValSTAFF);
        pfieldSTAFF.CurrentValues.Add(disValSTAFF);

        pfields.Add(pfieldFDATE);
        pfields.Add(pfieldTDATE);
        pfields.Add(pfieldSTAFF);

        CrystalReportViewer1.ParameterFieldInfo.Clear();
        CrystalReportViewer1.ParameterFieldInfo = pfields;

        report.Load(Server.MapPath("../SalesReport/ShippingSummByProductReport.rpt"));

        SPShipSummByProductTableAdapter adapter = new SPShipSummByProductTableAdapter();
        CrystalReportManager crManager = new CrystalReportManager(report, adapter.GetData(FromDate, ToDate));

        //Page.RegisterStartupScript("Alert", "<script>alert('From date: " + FromDate + " ');</script>");
        //Page.RegisterStartupScript("Alert", "<script>alert('To date: " + ToDate + " ');</script>");
        //Page.RegisterStartupScript("Alert", "<script>alert('Staff: " + fullName + " ');</script>");

        CrystalReportViewer1.ReportSource = crManager.Report;
        CrystalReportViewer1.Visible = true;
    }
    protected void CrystalReportViewer1_ViewZoom(object source, CrystalDecisions.Web.ZoomEventArgs e)
    {
        crvBinding();
    }
    protected void CrystalReportViewer1_Search(object source, CrystalDecisions.Web.SearchEventArgs e)
    {
        crvBinding();
    }
    protected void CrystalReportViewer1_Navigate(object source, CrystalDecisions.Web.NavigateEventArgs e)
    {
        crvBinding();
    }