i have a main form and a subform inside. the subform is a continous form connected to the main form by parent and child link

Question

I want my subform to have a sequential numbering on the left part and it will be dynamic depending on the loaded data from the relationship between the main form to subform by the parent and child link

I have this one it works fine.

    Public Function RowNum(frm As Form) As Variant
        On Error GoTo err_RowNum
        With frm.RecordsetClone
            .Bookmark = frm.Bookmark
            RowNum = .AbsolutePosition + 1
        End With
exit_RowNum:
            Exit Function
err_RowNum:
        If Err.Number <> 3021 Then 'Ignore "No bookmark" at new row
        Debug.Print " RowNum() error " & Err.Number * "-" & Err.Description
        End If
        RowNum = Null
        Resume exit_RowNum
    End Function

and on the subform on where the number will be shown I added unbound textbox and the control source I input this code =RowNum([Form]).

everything works good. unfortunately when I add new line below the last row, the number didn't update, it sticks to the last number and copy to the row where i am inputting new row.