Bulk SMS problem, not send all sms

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VB

Dear Friends/Experts,

I am trying a bulk sms send application using vb.net and windows 7, it sends one sms but not the others, but when I run the app step by step in debug mode it do send all smss, plz help, following is the potion of code used for send sms.

Button3.Enabled = False
      Dim c As Integer
      c = DataGridView1.RowCount - 1
      Dim r As Integer = 0
      Dim pducodes() As String
      Dim pduL As Integer
      Dim pduStr As String
      Dim Number As String
      If smsEngine.IsOpen = False Then
          smsEngine.Open()
      End If
      PBar1.Minimum = r
      PBar1.Maximum = c
      smsEngine.Write("AT" & vbCrLf)
      txtR.SelectedText = smsEngine.ReadLine & vbCrLf
      smsEngine.DiscardInBuffer()

      smsEngine.Write("AT+CMGF=0" & vbCrLf)
      'txtR.SelectedText = smsEngine.ReadLine & vbCrLf
      smsEngine.DiscardInBuffer()
      Label1.Visible = True
      Do
          PBar1.Value = r + 1
          Label1.Text = "Sending Message: " & r + 1 & "/" & c
          Number = DataGridView1.Item(0, r).Value
          DataGridView1.Rows(r).Selected = True
          If RdEng.Checked Then
              pducodes = GetPDU("+923189244444", Number, ENUM_TP_DCS.DefaultAlphabet, ENUM_TP_VALID_PERIOD.OneDay, 0, False, TextBox1.Text)
          Else
              pducodes = GetPDU("+923189244444", Number, ENUM_TP_DCS.UCS2, ENUM_TP_VALID_PERIOD.OneDay, 0, False, TextBox1.Text)
          End If
          pduStr = pducodes(0)
          pduL = (pducodes(0).Length - 14) / 2 - 1
          Application.DoEvents()
          smsEngine.Write("AT+CMGS=" & pduL & vbCr)
          txtR.SelectedText = smsEngine.ReadLine & vbCrLf
          Thread.Sleep(100)
          smsEngine.DiscardInBuffer()
          '  Do
          smsEngine.Write(pduStr & Chr(26))
          txtR.SelectedText = smsEngine.ReadLine & vbCrLf
          Thread.Sleep(200)
          Application.DoEvents()
          ' Loop While (InStr(smsEngine.ReadLine, "OK"))
          smsEngine.DiscardInBuffer()
          DataGridView1.Rows(r).Selected = False
          r = r + 1
          ' Thread.Sleep(2000)
          DataGridView1.Update()

      Loop While r < c
      smsEngine.Close()