I had a similar problem once. This is however Joomla's fault, not JSMF's.

Joomla and SMF have different rules when it comes to allowed characters and field lengths in usernames/real names. So if Joomla doesn't like the SMF username, the sync fails.

I got round the problem by editing usernames directly in the database. You could try that, or the more extreme solution of modifying Joomla files, as some users have done.

Personally, I'd just tell the specific users to change their usernames.