As this seems to be only one document I doubt it is anything to do with
spelling files but the formatting attribute decribed below.
In 2003 and prior:
1. Select all (Ctrl+A to select the whole document).
2. Double click on the language button at the bottom of the Word window (or
Tools, Language, Set Language).
3. Select Language required in top section of the dialog box.
4. Clear the "Do Not Check Spelling or Grammar" check box. While here also
clear the "Detect language automatically" if this is checked.
5. OK.
The document should now be open to spelling and grammar checking.
In Word 2007, Review tab, Proofing group, Thesaurus, Set Language.
Spell check is a formatting attribute of the text. If you copy formatted
text from another document then the formatting will come with it and if that
include the no proofing option, then that's what you get. You need to either
re-apply the proofing parameter (as described above) or use Paste Special  
Unformatted Text so that the formatting of the destination is adopted.
Also see http://sbarnhill.mvps.org/WordFAQs/MasterSpellCheck.htm.

Hope this helps, comes back if not.
DeanH