WM_VSCROLL message

I have a TListView, and I want to send a WM_VSCROLL message.
It can receive SM_LINEDOWN, but cannot receive SB_THUMBPOSITION or
THUMBTRACK
My code:

void __fastcall TForm1::Button1Click(TObject *Sender)
{
    static nPos;
    nPos++;
        WPARAM wParam = MAKEWPARAM(SB_THUMBTRACK, nPos);
        BOOL bRet = SendMessage(ListView1->Handle, WM_VSCROLL, wParam, 0);
    if (nPos == 20)
        nPos = 0;

Quote
}

Thanks
Shijun wei