Check box in a Word form not clearing

Hi,

I have a user who created a form and protected it to test the fields. After
testing, she unprotected it, selected the whole document, and hit F9 to clear
the test data. This worked great for almost every field. However, *some* of
the check boxes did not clear. We fixed it by reinserting a new check box.
Any ideas why some of the check boxes would clear after using the F9 key to
clear the data, and some of check boxes wouldn't clear?

I haven't seen this before, and couldn't find a post with the same problem.
She created two different forms (which are saved as templates in the 97-2003
format, although they were created in Word 2007, if that is helpful), and
both acted the same way.

Thanks for any tips!