The notes for a slide are kept in a separate object called NotesPage. The notes are available in a body-type placeholder shape. The following routine scans for this shape in the NotesPage of the specified slide and returns the notes for that slide.

Function GetNotesText(ByVal Sld As Slide) As String
    Dim Shp As Shape
    Dim S As String

    On Error Resume Next

    S = ""
    With Sld.NotesPage
        For Each Shp In .Shapes
            If Shp.Type = msoPlaceholder Then
                If Shp.PlaceholderFormat.Type = ppPlaceholderBody Then
                    S = Shp.TextFrame.TextRange.Text
                    Exit For
                End If
            End If
        Next
    End With

    GetNotesText = S
End Function