สอบถามเรื่อง การใช้ SerialPort vb.net 2005 ครับ เมื่อ run แล้วปุ่มอื่นไม่สามารถทำงานได้

   Private Sub frmAddRFID_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        'SerialPort1.Open()
        ' sp.Open()
        ListView1.Columns.Add("รหัสบัณฑิต", 80, HorizontalAlignment.Center)
        ListView1.Columns.Add("ชื่อ", 120, HorizontalAlignment.Center)
        ListView1.Columns.Add("Tag RFID", 95, HorizontalAlignment.Center)

        ListView1.View = View.Details
        ListView1.GridLines = True
        ListView1.FullRowSelect = True

        With Conn
            If .State = ConnectionState.Open Then .Close()
            .ConnectionString = strConn
            .Open()
        End With

        Dim sqlEducation As String = ""
        sqlEducation = "SELECT EdtID,EdtName FROM Education "
        sqlEducation &= " ORDER BY EdtID"

        Da = New SqlDataAdapter(sqlEducation, Conn)
        Da.Fill(Ds, "Education")

        If Ds.Tables("Education").Rows.Count <> 0 Then
            With cboEducation
                .DataSource = Ds.Tables("Education")
                .DisplayMember = "EdtName"
                .ValueMember = "EdtID"
            End With
        End If

        btnAdd.Enabled = True
        btnSave.Enabled = False
        btnCancle.Enabled = True
    End Sub
    Private Sub btnTagID_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnTagID.Click
        SerialPort1.Open()
        If SerialPort1.IsOpen Then
            With txtSeatTag
                .AppendText(Mid(SerialPort1.ReadLine & vbCrLf, 2, 10))
                .ScrollToCaret()
            End With
        End If
        btnAdd.Enabled = True
        SerialPort1.Close()
    End Sub
    Private Sub txtSeatTag_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles txtSeatTag.TextChanged

        txtSeatTag.Text = ""
        If SerialPort1.IsOpen Then

            SerialPort1.Close()
            SerialPort1.Open()

            With txtSeatTag
                .AppendText(Mid(SerialPort1.ReadLine & vbCrLf, 2, 10))
                .ScrollToCaret()
            End With

            NextGraduate()

        End If
        SerialPort1.Close()
    End Sub

    Private Sub NextGraduate()

        Me.BindingContext(Ds, "Graduate").Position += 1
        ShowPosition()
        Me.cboEducation.Enabled = False

        Dim i As Integer = 0
        Dim lvi As ListViewItem
        Dim tmpGraduate As Integer = 0

        For i = 0 To ListView1.Items.Count - 1
            tmpGraduate = Val(ListView1.Items(i).SubItems(1).Text)
            If Val(txtSeatTag.Text) = tmpGraduate Then
                MessageBox.Show("คุณใส่ชื่อบัณฑิตซ้ำ กรุณาใส่ใหม่ !!!", "ผลการตรวจสอบ", MessageBoxButtons.OK, MessageBoxIcon.Information)
                ClearAllData()

                Exit Sub
            End If
        Next

        Dim anyData() As String
        anyData = New String() { _
            lblGdtID.Text, _
            lblGdtFname.Text, _
            txtSeatTag.Text _
        }

        lvi = New ListViewItem(anyData)
        ListView1.Items.Add(lvi)

        ClearAllData()

    End Sub