How do I resolve the spreadsheet locked for editing error?

Using Office 2010, I have a PowerPoint presentation where all charts are linked to an Excel workbook. To move a presentation and / or workbook to a different directory, all links must be updated to point to the location of the new book. To do this, I wrote the following code, which is located in the standard code module in PowerPoint:

Private Sub RedirectLinks()

  Dim Source As String
  Dim Dest As String
  Dim Action As Integer
  If InStr(1, ActivePresentation.Path, "Dev\") > 1 Then
    Action = MsgBox("Changing pointers to PRODUCTION", vbOKCancel)
    Source = "Dev\"
    Dest = vbNull
  Else
    Action = MsgBox("Changing pointers to DEVELOPMENT", vbOKCancel)
    Source = "Templates\"
    Dest = "Dev\Templates\"
  End If

  If Action = vbOK Then
    Dim SL As Slide
    Dim SH As Shape
    Dim Top As Double
    Dim Left As Double
    Dim Width As Double
    Dim Height As Double
    For Each SL In ActivePresentation.Slides
      SL.Select
      For Each SH In SL.Shapes
        SH.Select
        If SH.Type = msoLinkedOLEObject Then                                 'when we find a linked one
          Top = SH.Top
          Left = SH.Left
          Width = SH.Width
          Height = SH.Height
          SH.LinkFormat.SourceFullName = Replace(SH.LinkFormat.SourceFullName, Source, Dest)
          SH.Top = Top
          SH.Left = Left
          SH.Height = Height
          SH.Width = Width
        End If
      Next
    Next
  End If

  If InStr(1, Dest, "dev") > 0 Then
    Action = MsgBox("About to OVER WRITE the Dev copy with this one." & vbCrLf & "Click 'Cancel' to prevent this and save manually", vbOKCancel, "OVER WRITE WARNING!!")
  Else
    Action = MsgBox("About to OVER WRITE the PRODUCTION copy with this one." & vbCrLf & "Click 'Cancel' to prevent this and save manually", vbOKCancel, "OVER WRITE WARNING!!")
  End If

  If Action = vbOK Then
    ActivePresentation.SaveAs Replace(ActivePresentation.Path, Source, Dest) & ActivePresentation.Name
  End If

End Sub

      

        
        
        
      

    

The code runs just fine, however I often get this message box from Excel when it executes a line SH.LinkFormat.SourceFullName = Replace(SH.LinkFormat.SourceFullName, Source, Dest)

.

Notes:

• Actually this book is closed - I know it hasn't been opened by anyone else (I'm the only one who usually uses it and another person who isn't there in the office this morning).
• It claims the file is locked 'another user'
  
  , which is actually me. I can often get this warning by closing the workbook and then immediately reopening it. I don't know if the problem is caused by network latency (the file is on the server, not locally) or something, but a few seconds after using the workbook I get the message workbook is now available for read-write
  
  .
• I don't get this warning every time it tries to execute the line that sets .SourceFullName
  
  . Sometimes I get it most of the time, sometimes I don't get it at all, sometimes I get it sometimes.
• Despite my thoughts on network lag, no matter how fast or slow I debug the code, I get this message in random order.
• Marking new or old books as Read-only
  
  OS level doesn't seem to make things better.
  • However, tagging both seems to give me 2 warnings for every replacement is done.

Does anyone have any suggestions to fix this?