From allexperts.com:
There are a few things you can check
on:
Make sure that the second table doesn't have any rows marked as
heading rows.
Make sure that neither table is wrapped (wrapping should be set to
None on the Table tab of Table
Properties).
Make sure that neither table (even if they appear identical) is nested in
one large cell of a containing table
(this sometimes happens with material
pasted from the Web).
That said, I can tell you that I once
had two tables--which I had created
myself, so I know there was nothing
unusual about them--that just refused
to merge, for no apparent reason. It's
possible that the table structures
were somehow damaged, and if I'd been
doing this in Word 2003, perhaps using
Open and Repair would have fixed the
problem. As it was, it wasn't vital
that the tables be actually part of
the same table, so I shrugged and
moved on.
If you encounter such a situation, you
have really only two recourses (if
Open and Repair doesn't help):
Add rows to the first table and copy/paste the content of the second
table into them.
Convert both tables to text, then convert all the text back to a single
table.
Sorry I can't be more definitive, but
this is a mystery to me, too!
Suzanne S. Barnhill
Microsoft MVP (Word) 1998-2006