win api sendkeys doesn't work on a specific programm - vb.net

Question

I want to send a leftbutton-mouseclick to a (specific) program but it doesn't work. when i tried my code on several other apps it worked without any major changes.
this is my Declaration:
Public Class Win32
#Region "Methods And Functions Declaration"
Public Declare Function FindWindow Lib "user32.dll" Alias "FindWindowA" (ByVal IpClassName As String, ByVal IpWindowName As String) As Int32
Public Declare Function SetForegroundWindow Lib "user32.dll" (ByVal hwnd As Int32) As Int32
Public Declare Function FindWindowEx Lib "user32.dll" Alias "FindWindowExA" (ByVal hWnd1 As Int32, ByVal hWnd2 As Int32, ByVal lpsz1 As String, ByVal lpsz2 As String) As Int32
Public Declare Function EnumChildWindows Lib "user32.dll" (ByVal hWndParent As Int32, ByVal lpEnumFunc As Int32, ByVal lParam As Int32) As Int32
Public Declare Function SendMessage Lib "user32.dll" Alias "SendMessageA" (ByVal hwnd As Int32, ByVal wMsg As Int32, ByVal wParam As Int32, ByRef lParam As Int32) As Int32
Public Declare Function SendMessage Lib "user32.dll" Alias "SendMessageA" (ByVal hwnd As Int32, ByVal wMsg As Int32, ByVal wParam As Int32, ByRef lParam As String) As Int32
#End Region
#Region "Messages"
Public Const BM_CLICK As Int32 = &HF5
Public Const WM_NCLBUTTONDOWN As Int32 = &HA1
Public Const WM_NCHITTEST As Int32 = &H84
Public Const WM_NCLBUTTONDBLCLK As Int32 = &HA3
Public Const WM_NCLBUTTONUP As Int32 = &HA2
Public Const WM_SETFOCUS As Long = &H7
Public Const WM_SETTEXT As Long = &HC
Public Const WM_NCRBUTTONDOWN As Int32 = &HA4
#End Region
the actual code:
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Druck Eingeben
Dim Hwnd As Integer = Win32.FindWindow(Nothing, "V1.22 BMTS Prüfstand 2")
If (Not Hwnd = 0) Then
Dim HwndInfo As Integer = Win32.FindWindowEx(Hwnd, Nothing, "TGroupBox", "Info")
If Not HwndInfo = 0 Then
Dim ccHwnd As Integer = Win32.FindWindowEx(HwndInfo, Nothing, "TPanel", Nothing)
If Not ccHwnd = 0 Then
Dim cccHwnd As Integer = Win32.FindWindowEx(ccHwnd, Nothing, "Tedit", Nothing)
If Not cccHwnd = 0 Then
Win32.SetForegroundWindow(cccHwnd)
SendKeys.Send("990")
Else
MessageBox.Show("Bitte Starte erst die Programme")
End If
Else
MessageBox.Show("Bitte Starte erst die Programme")
End If
Else
MessageBox.Show("Bitte Starte erst die Programme")
End If
Win32.SetForegroundWindow(HwndInfo)
SendKeys.Send("999")
Else
MessageBox.Show("Bitte Starte erst die Programme")
End If
'Startbutton Klicken
Dim HwndFunktion As Integer = Win32.FindWindowEx(Hwnd, Nothing, "TGroupBox", "Funktion")
If Not HwndFunktion = 0 Then
Dim cHwnd As Integer = Win32.FindWindowEx(HwndFunktion, Nothing, "TButton", "S T A R T")
If Not cHwnd = 0 Then
Win32.SetForegroundWindow(cHwnd)
Win32.SendMessage(cHwnd, Win32.WM_SETFOCUS, 0, 0)
Win32.SendMessage(cHwnd, Win32.WM_NCLBUTTONDBLCLK, 0, 0)
Win32.SendMessage(cHwnd, Win32.WM_NCLBUTTONDBLCLK, 0, 0)
Win32.SendMessage(cHwnd, Win32.WM_NCLBUTTONDOWN, 0, 0)
Win32.SendMessage(cHwnd, Win32.WM_SETTEXT, 0, "6")
Else
MessageBox.Show("Bitte Starte erst die Programme")
End If
Else
MessageBox.Show("Bitte Starte erst die Programme")
End If
End Sub
To the Program itself: It runs on an old test bench, on win 7.
I think it depends on .net 3.5 but i don't have detailed information about it.
As a test, i filled a textbox with my code and even changed the label of the button. Just the click function doesn't work.
First I tried in with mouse positioning, now directly with the handles.
It doesn't work either way.
Please remember, the code works fine on a few tested Programs (except the one I need it for).