How to best count the checked checkboxes in a datagrid after each selection/deselection

I am trying to keep a running total of the number of checkboxes selected in a datagrid. My problem is not in the coding but how to invoke the code after each selection/deselection?

The default event for the checkbox is
Protected Sub CheckBox_Select_CheckedChanged(ByVal sender As Object, ByVal e As System.EventArgs)

however this event is not triggered when a checkbox is changed.

any clues to why this would not be trigerred? could it be something to do with masterpages? or being inside an accordion control

' This code works if I request it from a button but the 
 
    Protected Sub CheckBox_Select_CheckedChanged(ByVal sender As Object, ByVal e As System.EventArgs)
        Dim cb As CheckBox
        Dim row As GridViewRow
        Dim contactCounter As Integer = 0
 
        For Each row In GridView1.Rows
            cb = CType(row.FindControl("CheckBox_Select"), CheckBox)
            If cb.Checked Then
                contactCounter = contactCounter + 1
            End If
        Next
        Label_contactsSelected.Text = contactCounter & " contacts selected"
    End Sub

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