I don't think Activate will do anything for you.

If calculation mode is automatic then, if the links are correctly formed, the
updates should all occur on opening the master workbook.

Might be worth including
 Application.Calculate
in the procedure just to be sure of the first.

So we are left with the links being badly formed, in the sense that Excel does
not realise that they are addressing the other workbook.

You are probably on the right track with UNC vs drivename filenames.
Links to files in the same directory are effectively held as relative links (ie
they should work regardless of the path used to access that directory).
However, if one workbook thinks it came from \\Server\dir and the other thinks
it came from G:\dir they may not be able to tell that the directories are the
same and thereby make the links between the two open workbooks.

I would suggest changing the Open line to
   Workbooks.Open Filename:= _
       ThisWorkbook.Path & "\Master.xls", UpdateLinks:=3
which will ensure that both workbooks think they were loaded from the same
directory.

If the links in one or other file are to the wrong form of directory they may
need to be updated.  For example, in the data entry workbook (which only has one
link source?).. something like this (untested):
 Dim V
 V = ThisWorkbook.LinkSources(xlExcelLinks)
 If LCase(V(LBound(V)))<>LCase(ThisWorkbook.Path & "\Master.xls") Then
   ThisWorkbook.ChangeLink V(LBound(V)), ThisWorkbook.Path & "\Master.xls"
 End If
 
For the links in the master it is a bit more complicated because there are
multiple sources.
 Dim I As Integer
 V = Workbooks("Master.xls").LinkSources(xlExcelLinks)
 For I=LBound(V) To UBound(V)
   If LCase(Right(V(I),Len(ThisWorkbook.Name)+1))="\" &
LCase(ThisWorkbook.Name) Then
     Workbooks("Master.xls").ChangeLink V(I), ThisWorkbook.FullName
   End If
 Next

Bill Manville
MVP - Microsoft Excel, Oxford, England
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