Several people have had trouble with HTML special characters in their "spelled-out" versions.
Have you tried a numeric enconding for the same symbol? My Windows charmap shows a character reference of U+221A (I think this translates to UTF-8 0x221A); a site called webref.info shows an HTML encoding of √. The HTML numeric seems more likely, but I think that RH went UTF-8 compliant in version RH7 or 8.
You could also try select/copy/paste from Windows charmap - I don't know if that will work better, but RoboHelp is fairly Windows-oriented.
Sorry I can't be more specific, but I have an older version of RH and can't test these suggestions.