One possibility is that the path is too long to store the headers for that folder (i.e. the path to the folders *.msf file is too long) though I'd hope it would show up in the list of folders to subscribe even if it couldn't be subscribed (due to the path problem). I suggest you enable IMAP logging per
http://kb.mozillazine.org/Session_logging_for_mail/news and see if that folder is returned by the IMAP server. Just look for storage and report in the log file.
See
http://kb.mozillazine.org/Moving_your_p ... hunderbird if you want to move your profile to a different location, that has a short path. For example, I store my Thunderbird and Firefox profiles within c:\users\eric\profiles.
You could also uncheck the checkbox for "show only subscribed folders" in the account's server setting -> advanced section. You might not want to do that permanently, but it would help troubleshoot whats going on.