I've narrowed it down to this. It's probably a mysql setting (minimum word length that get indexed). Problem is... if on shared hosting, provider probably won't change it to 3. According to my information default is 4. Would be nice if it could be set from SS_config though.
@guy - yes that's not really fixable on shared hosting.
I've noticed that on MySQL 5.6, where you can do a fulltext search on InnoDB tables, in some cases a minimum of 3 character seems to be enabled by default for some reason... so if you're on 5.6, and you're still using MyISAM you could try converting tables to innoDB...
I think PeterLu's problem is different, because he's searching on 'course' and still not finding his page.