I have a series of documents that all had the same problem. I had to take
the following steps to make the header row repeat:
First, cut the table out of the original document.
Open a new document.
Edit -> Paste Special -> Formatted Text. (The table should go into the new
document and the header row should repeat properly. If not, go have a beer
and curse BG.)
Copy the table from the new document.
Paste it back into the original document, once again pasting it as formatted
text.
That was the only solution I found when the header rows refused to repeat.