I have a workbook with two worksheets, each one with a table and I'm trying
to get a macro to run that filters the the second worksheet table with the
value of the the cell selected in column A of the first worksheet. After
getting some hints here on this forum I came up with:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

Windows("Inventory Items.xlsm:1").Activate

If ActiveCell.Column = 1 Then
Dim ItemNum As String
ItemNum = "=" & ActiveCell.Text
Windows("Inventory Items.xlsm:2").Activate
ActiveSheet.ListObjects("Table_Default__ipurch").R ange.AutoFilter
Field:=1, _
Criteria1:=ItemNum, Operator:=xlAnd
Windows("Inventory Items.xlsm:2").Activate
End If
End Sub

but it doesn't work automatically when I select another cell (in the first
column)even though it runs perfectly when I run it manually with the first
line changed to:

Sub FilterPurchases()

How does one get it to run automatically when the selection changes?