It's possible the document has 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.

It's also possible a fault has developed in your Office installation. You might try repairing it (via Windows Control Panel > Programs > Programs & Features > Microsoft Office (version) > Change > Repair).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]