We recently encountered two situation where the user could not check spelling in a document that was originally created by someone else. When running Spell Check, they received a message that said "The spelling and grammar check is complete. Text marked with 'Do not check spelling or grammar' was skipped.
This is what we did to fix the Spell and Grammar check:
In Word 2003, click Tools > Language and select "Set Language".
In Word 2007, click the Review tab and in the Proofing section, click Set Language.
Under the box that lists languages, uncheck the checkbox that says "Do not check spelling and grammar"