Correct the protected
Unfortunately, Word's protection feature disables a huge number of important functions, even if you only protect a single section of a document.
Besides the spellchecker, many other items on the View, Insert, Format, Tools and Table menus are disabled, as well as most items on the Drawing, Database, Visual Basic and Picture toolbars.
This MS Word MVP FAQ Site article lays out the steps needed to
Enable the spellchecker in a protected document.
One of the important elements is "NoReset=True". The act of removing and then re-protecting a document will reset the data without this line.
In Office 2007+ Protection is found on the Review tab:
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