Excel 2013 - Macro to import sheet from another wb not working.

Hi All,
So my problem is I have this existing macro to pull a sheet from another workbook in to the active workbook. All works perfectly in 2003, I have just upgraded to 2013 and it won't work, keeps saying Can't find the file, but if I open it in 2003 it does it perfectly.

Sub ImportWorksheet()
Sheets("Approval").Select
PathName = Range("C18").Value
Filename = Range("C20").Value
TabName = Range("C19").Value

    ControlFile = ActiveWorkbook.Name
    Workbooks.Open Filename:=PathName & Filename
    
    ActiveSheet.Name = TabName
    Sheets(TabName).Copy After:=Workbooks(ControlFile).Sheets(1)
    
    Windows(Filename).Activate
    ActiveWorkbook.Close SaveChanges:=False
    Windows(ControlFile).Activate

End Sub

Any help getting this working would be great. I have searched the forumns but have come up empty handed.
Thanks,