VB and the Taskbar/System Tray

I've got an app that's trying to minimize itself to the taskbar/system tray (whatever you want to call it), but the callbacks on WM_MOUSEMOVE aren't working. Here's the code I use in the Form_Load:

    nid.cbSize = Len(nid)
    nid.hwnd = Me.hwnd
    nid.uId = vbNull
    nid.uFlags = NIF_ICON Or NIF_TIP Or NIF_MESSAGE
    nid.uCallBackMessage = WM_MOUSEMOVE
    nid.hIcon = Me.Icon
    If (False = Shell_NotifyIcon(NIM_ADD, nid)) Then
        MsgBox "Oops!"
    End If
   
    If (gs.p.fMinimiseOnStartup) Then
        frmMain.WindowState = vbMinimized
        frmMain.Hide
    End If

And here's the procedure that SHOULD be getting the callbacks:
Private Sub Form_MouseMove(Button As Integer, _
                           Shift As Integer, _
                           X As Single, _
                           Y As Single)
    Dim Result As Long
    Dim msg As Long
   
    If frmMain.ScaleMode = vbPixels Then
        msg = X
    Else
        msg = X / Screen.TwipsPerPixelX
    End If
    Select Case msg
        Case WM_LBUTTONUP                    
            frmMain.WindowState = vbNormal
            SetForegroundWindow (frmMain.hwnd)
            frmMain.Show
        Case WM_LBUTTONDBLCLK                
            frmMain.WindowState = vbNormal
            SetForegroundWindow frmMain.hwnd
            frmMain.Show
        Case WM_RBUTTONUP
            SetForegroundWindow frmMain.hwnd
            frmMain.PopupMenu frmMain.mPopupSys
    End Select
End Sub

But it's not getting any MOUSEMOVE messages from the icon. The icon appears in the system tray just fine, and if I leave the form up, the MOUSEMOVE messages are invoking the procedure above...but any mouse movement over the icon in the system tray doesn't. What am I doing wrong?