There may, of course, be an issue with the way you're automating Word, rather than with Word itself. In any event, since you're automating Word, you might do better to save the document as a normal document (with mergefields, etc. in place) and add the mailmerge process to the automation code itself. For example:
Code:
Sub RunMerge()
' Demo code for a vba-driven mailmerge.
Dim wdApp As New Word.Application
Dim wdDoc As Word.Document
wdApp.DisplayAlerts = wdAlertsNone
Dim StrSrc As String, StrDoc As String
StrDoc = "C:\Users\" & Environ(UserName) & "\Downloads\MergeDoc.doc"
StrSrc = "C:\Users\" & Environ(UserName) & "\Downloads\DataFile.csv"
Set wdDoc = wdApp.Documents.Open(StrDoc)
With wdDoc
With .MailMerge
.MainDocumentType = wdFormLetters
.OpenDataSource _
Name:=StrSrc, ReadOnly:=True, AddToRecentFiles:=False, LinkToSource:=False, _
Connection:="", SQLStatement:="", SQLStatement1:="", SubType:=wdMergeSubTypeOther
.Destination = wdSendToNewDocument
.SuppressBlankLines = True
With .DataSource
.FirstRecord = wdDefaultFirstRecord
.LastRecord = wdDefaultLastRecord
End With
.Execute Pause:=False
.MainDocumentType = wdNotAMergeDocument
End With
.Close False
End With
wdApp.DisplayAlerts = wdAlertsAll
End Sub