I'm having the same problem. It appears that only the first instance and instances which have been modified individually show up in the search.
As a musician, I may have a number of rehearsals for one gig spread across several months. Rather than scanning each month visually one by one, I'd rather search for "[x] rehearsal" to count the number of rehearsals. That's just one example.
Now that I know iCal works this way, in the future I will take the time to add each event individually rather than taking the shortcut of using repeats... unless Apple decides to fix this or offer it as a preference. Honestly, I can't think of any situations where I *wouldn't* want all instances to show up in a search.