FindWindowEx() does not always return child window controls

There was a task to control separate application via button clicks. We've created a small prototype, that can identify top-level window, and after that found necessary control inside this window using FindWindowEx. To my surprise it was not always working as I expected. Sometimes I was getting null instead of handle to the corresponding window. So I decided to create my own version of that method:

private static IntPtr WaitChild(IntPtr parent, string windowClass,
                                string windowCaption) {
 IntPtr result = IntPtr.Zero;
 do {
     Thread.Sleep(10);
     EnumChildWindows(parent, delegate(IntPtr hWnd, IntPtr lParam) {
                              StringBuilder className = new StringBuilder(200);
                              GetClassName(hWnd, className, 200);
                              StringBuilder caption = new StringBuilder(200);
                              GetWindowText(hWnd, caption, 200);
                              if (className.ToString() == windowClass &&
                                  caption.ToString() == windowCaption)
                                  result = hWnd;
                              return true;
                          }, IntPtr.Zero);
 } while (result == IntPtr.Zero);
 return result;
}

[DllImport("user32.dll")]
private static extern bool EnumChildWindows(IntPtr hwndParent,
                           EnumWindowsProc lpEnumFunc, IntPtr lParam);

private delegate bool EnumWindowsProc(IntPtr hWnd, IntPtr lParam);

[DllImport("user32.dll", SetLastError=true, CharSet=CharSet.Auto)]
private static extern int GetWindowText(IntPtr hWnd,
                       [Out] StringBuilder lpString, int nMaxCount);

[DllImport("user32.dll", CharSet=CharSet.Auto)]
private static extern int GetClassName(IntPtr hWnd,
                       StringBuilder lpClassName, int nMaxCount);

Sometimes it takes one or two loops while the window is finally appears on the screen, but it works pretty stable now