If you copy & paste a table from Excel or a website, etc., only Word's 'Normal' Table Styles is applied, so modifying one or more other Table Styles wouldn't resolve the issue unless you then apply one - which might also mess with the format of what you've just pasted. Even copying & pasting from another document using the same Table Styles as you applied the 'heading row' attribute to in the destination document won't necessarily apply that attribute to the pasted table.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]