OK, you aren't missing anything because Word just does not work the way you
are trying to. Using the out-of-the-box facilities, you can't keep the
existing data source open in a way that means it is used as the data source
for a new document you create.
I'm gueesing that what you're trying to do is have merge documents A.doc,
B.doc, C.doc etc. and attach them each day (say) to a new data source
(perhaps 0420.xls today, 0421.xls tomorrow, etc.
If so, it would be easier to attach a.doc, b.doc, c.doc to a single data
source, say Z.xls, then when you want to do your merges
a. close any mail merge main documents that might be open
b. delete or rename z.xls
c. copy today's data source to z.xls
d. do your merges.
You /might/ be able to do things the from within the mail merge manager by
a. creating a new blank document (say x.doc) and attaching it to today's
data source (say 0420.xls)
b. using insert|file to insert a.doc. I think (but have not checked) that
the document wil remain attached to 0420.xls
c. do the merge
d. delete the contents of x.doc
e. use insertfile to insert b.doc
f. do the merge
g. etc...
However, I think that is not likely to work well because of problems with
document layout, headers and footers, and so on.
Peter Jamieson