In that case, your mailmerge main document may have acquired some of corruption. Corrupt documents can often be 'repaired' by inserting a new, empty, paragraph at the very end, copying everything except that new paragraph to a new document based on the same template (headers & footers may need to be copied separately), closing the old document and saving the new one over it. With a mailmerge main document, you'd also need to re-connect to the data source and re-apply any filtering.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]