SendMessage not working

Question

I am trying to automate a login process. I need to pass 2 times tab and a keydown(for a drop down) and enter event in order to login. I am using below code

 parenthwnd = FindWindow1(vbNullString, "APPName")
  retvalue = SendMessage(parenthwnd, WM_KEYDOWN, VK_TAB, 0)
  retvalue = SendMessage(parenthwnd, WM_KEYDOWN, VK_TAB, 0)     
  retvalue = SendMessage(parenthwnd, WM_KEYDOWN, VK_DOWN, 0)
  retvalue = SendMessage(parenthwnd, WM_KEYUP, VK_DOWN, 0)
  retvalue = PostMessage(parenthwnd, BM_CLICK, 0, 0)

While the tab events are working. I am not able to get the keydown and enter event to work. Is there something I am missing.