VBA- Works when cell value changes (manually) but not when cell formula value changes

As title suggests, the below VBA code only works when the cell value is changed when a value is entered manually and not when the value changes as a result of a simple =SUM formula;

Private Sub Worksheet_Change(ByVal Target As Range)
Select Case Target.Address
Case "$W$18"
    If Target.Value > 0 Then
        Chk_2On
    Else
        Chk_2Off
    End If
End Select
End Sub

Sub Chk_2On()
ActiveSheet.CheckBoxes("Check Box 59").Visible = True
End Sub
Sub Chk_2Off()
ActiveSheet.CheckBoxes("Check Box 59").Visible = False
End Sub

Cell $W$18 contains the =SUM formula.

Thanks for the help in advance.

Glen